Source complete env in nix-shell with __structuredAttrs = true;

This is needed to push the adoption of structured attrs[1] forward. It's
now checked if a `__json` exists in the environment-map of the derivation
to be openend in a `nix-shell`.

Derivations with structured attributes enabled also make use of a file
named `.attrs.json` containing every environment variable represented as
JSON which is useful for e.g. `exportReferencesGraph`[2]. To
provide an environment similar to the build sandbox, `nix-shell` now
adds a `.attrs.json` to `cwd` (which is mostly equal to the one in the
build sandbox) and removes it using an exit hook when closing the shell.

To avoid leaking internals of the build-process to the `nix-shell`, the
entire logic to generate JSON and shell code for structured attrs was
moved into the `ParsedDerivation` class.

[1] https://nixos.mayflower.consulting/blog/2020/01/20/structured-attrs/
[2] https://nixos.org/manual/nix/unstable/expressions/advanced-attributes.html#advanced-attributes

src/libstore/parsed-derivations.hh

@@ -6,6 +6,8 @@
 
 namespace nix {
 
+typedef std::pair<std::string, nlohmann::json> StructuredAttrsWithShellRC;
+
 class ParsedDerivation
 {
     StorePath drvPath;
@@ -36,6 +38,8 @@ public:
     bool willBuildLocally(Store & localStore) const;
 
     bool substitutesAllowed() const;
+
+    std::optional<StructuredAttrsWithShellRC> generateStructuredAttrs(std::optional<StringMap> inputRewrites, Store & store, const StorePathSet & inputPaths);
 };
 
 }