Move value-only methods to InstallableValue

These methods would previously fail on the other `Installable`s, so
moving them to this class is more correct as to where they actually
work.

Additionally, a `InstallableValueCommand` is created to make it easier
(or rather no worse than before) to write commands that just work on
`InstallableValue`s.

Besides being a cleanup to avoid failing default methods, this gets us
closer to https://github.com/NixOS/rfcs/pull/134.

src/libcmd/installables.hh

@@ -18,19 +18,6 @@ struct SourceExprCommand;
 
 namespace eval_cache { class EvalCache; class AttrCursor; }
 
-struct App
-{
-    std::vector<DerivedPath> context;
-    Path program;
-    // FIXME: add args, sandbox settings, metadata, ...
-};
-
-struct UnresolvedApp
-{
-    App unresolved;
-    App resolve(ref<Store> evalStore, ref<Store> store);
-};
-
 enum class Realise {
     /* Build the derivation. Postcondition: the
        derivation outputs exist. */
@@ -92,13 +79,6 @@ struct Installable
 
     DerivedPathWithInfo toDerivedPath();
 
-    UnresolvedApp toApp(EvalState & state);
-
-    virtual std::pair<Value *, PosIdx> toValue(EvalState & state)
-    {
-        throw Error("argument '%s' cannot be evaluated", what());
-    }
-
     /* Return a value only if this installable is a store path or a
        symlink to it. */
     virtual std::optional<StorePath> getStorePath()
@@ -106,16 +86,6 @@ struct Installable
         return {};
     }
 
-    /* Get a cursor to each value this Installable could refer to. However
-       if none exists, throw exception instead of returning empty vector. */
-    virtual std::vector<ref<eval_cache::AttrCursor>>
-    getCursors(EvalState & state);
-
-    /* Get the first and most preferred cursor this Installable could refer
-       to, or throw an exception if none exists. */
-    virtual ref<eval_cache::AttrCursor>
-    getCursor(EvalState & state);
-
     virtual FlakeRef nixpkgsFlakeRef() const
     {
         return FlakeRef::fromAttrs({{"type","indirect"}, {"id", "nixpkgs"}});