Use std::set<StringContextElem> not PathSet for string contexts

Motivation

`PathSet` is not correct because string contexts have other forms
(`Built` and `DrvDeep`) that are not rendered as plain store paths.
Instead of wrongly using `PathSet`, or "stringly typed" using
`StringSet`, use `std::std<StringContextElem>`.

-----

In support of this change, `NixStringContext` is now defined as
`std::std<StringContextElem>` not `std:vector<StringContextElem>`. The
old definition was just used by a `getContext` method which was only
used by the eval cache. It can be deleted altogether since the types are
now unified and the preexisting `copyContext` function already suffices.

Summarizing the previous paragraph:

Old:

  - `value/context.hh`: `NixStringContext = std::vector<StringContextElem>`
  - `value.hh`: `NixStringContext Value::getContext(...)`
  - `value.hh`: `copyContext(...)`

New:

  - `value/context.hh`: `NixStringContext = std::set<StringContextElem>`
  - `value.hh`: `copyContext(...)`
----

The string representation of string context elements no longer contains
the store dir. The diff of `src/libexpr/tests/value/context.cc` should
make clear what the new representation is, so we recommend reviewing
that file first. This was done for two reasons:

Less API churn:

`Value::mkString` and friends did not take a `Store` before. But if
`NixStringContextElem::{parse, to_string}` *do* take a store (as they
did before), then we cannot have the `Value` functions use them (in
order to work with the fully-structured `NixStringContext`) without
adding that argument.

That would have been a lot of churn of threading the store, and this
diff is already large enough, so the easier and less invasive thing to
do was simply make the element `parse` and `to_string` functions not
take the `Store` reference, and the easiest way to do that was to simply
drop the store dir.

Space usage:

Dropping the `/nix/store/` (or similar) from the internal representation
will safe space in the heap of the Nix programming being interpreted. If
the heap contains many strings with non-trivial contexts, the saving
could add up to something significant.

----

The eval cache version is bumped.

The eval cache serialization uses `NixStringContextElem::{parse,
to_string}`, and since those functions are changed per the above, that
means the on-disk representation is also changed.

This is simply done by changing the name of the used for the eval cache
from `eval-cache-v4` to eval-cache-v5`.

----

To avoid some duplication `EvalCache::mkPathString` is added to abstract
over the simple case of turning a store path to a string with just that
string in the context.

Context

This PR picks up where #7543 left off. That one introduced the fully
structured `NixStringContextElem` data type, but kept `PathSet context`
as an awkward middle ground between internal `char[][]` interpreter heap
string contexts and `NixStringContext` fully parsed string contexts.

The infelicity of `PathSet context` was specifically called out during
Nix team group review, but it was agreeing that fixing it could be left
as future work. This is that future work.

A possible follow-up step would be to get rid of the `char[][]`
evaluator heap representation, too, but it is not yet clear how to do
that. To use `NixStringContextElem` there we would need to get the STL
containers to GC pointers in the GC build, and I am not sure how to do
that.

----

PR #7543 effectively is writing the inverse of a `mkPathString`,
`mkOutputString`, and one more such function for the `DrvDeep` case. I
would like that PR to have property tests ensuring it is actually the
inverse as expected.

This PR sets things up nicely so that reworking that PR to be in that
more elegant and better tested way is possible.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>

src/libexpr/eval.hh

@@ -56,7 +56,7 @@ void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env &
 
 std::unique_ptr<ValMap> mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env & env);
 
-void copyContext(const Value & v, PathSet & context);
+void copyContext(const Value & v, NixStringContext & context);
 
 
 /**
@@ -327,7 +327,7 @@ public:
      * intended to distinguish between import-from-derivation and
      * sources stored in the actual /nix/store.
      */
-    Path toRealPath(const Path & path, const PathSet & context);
+    Path toRealPath(const Path & path, const NixStringContext & context);
 
     /**
      * Parse a Nix expression from the specified file.
@@ -423,7 +423,7 @@ public:
      */
     void forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx);
     std::string_view forceString(Value & v, const PosIdx pos, std::string_view errorCtx);
-    std::string_view forceString(Value & v, PathSet & context, const PosIdx pos, std::string_view errorCtx);
+    std::string_view forceString(Value & v, NixStringContext & context, const PosIdx pos, std::string_view errorCtx);
     std::string_view forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx);
 
     [[gnu::noinline]]
@@ -439,7 +439,7 @@ public:
     bool isDerivation(Value & v);
 
     std::optional<std::string> tryAttrsToString(const PosIdx pos, Value & v,
-        PathSet & context, bool coerceMore = false, bool copyToStore = true);
+        NixStringContext & context, bool coerceMore = false, bool copyToStore = true);
 
     /**
      * String coercion.
@@ -449,12 +449,12 @@ public:
      * booleans and lists to a string.  If `copyToStore` is set,
      * referenced paths are copied to the Nix store as a side effect.
      */
-    BackedStringView coerceToString(const PosIdx pos, Value & v, PathSet & context,
+    BackedStringView coerceToString(const PosIdx pos, Value & v, NixStringContext & context,
         std::string_view errorCtx,
         bool coerceMore = false, bool copyToStore = true,
         bool canonicalizePath = true);
 
-    StorePath copyPathToStore(PathSet & context, const Path & path);
+    StorePath copyPathToStore(NixStringContext & context, const Path & path);
 
     /**
      * Path coercion.
@@ -463,12 +463,12 @@ public:
      * path.  The result is guaranteed to be a canonicalised, absolute
      * path.  Nothing is copied to the store.
      */
-    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
+    Path coerceToPath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx);
 
     /**
      * Like coerceToPath, but the result must be a store path.
      */
-    StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
+    StorePath coerceToStorePath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx);
 
 public:
 
@@ -573,6 +573,12 @@ public:
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, PosIdx pos);
 
+    /* Create a string representing a store path.
+
+       The string is the printed store path with a context containing a single
+       `Opaque` element of that store path. */
+    void mkStorePathString(const StorePath & storePath, Value & v);
+
     void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
 
     /**
@@ -584,7 +590,7 @@ public:
      * Realise the given context, and return a mapping from the placeholders
      * used to construct the associated value to their final store path
      */
-    [[nodiscard]] StringMap realiseContext(const PathSet & context);
+    [[nodiscard]] StringMap realiseContext(const NixStringContext & context);
 
 private: