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/store-api.hh

@@ -695,6 +695,8 @@ public:
 
     const Stats & getStats();
 
+    StorePathSet exportReferences(const StorePathSet & storePaths, const StorePathSet & inputPaths);
+
     /* Return the build log of the specified store path, if available,
        or null otherwise. */
     virtual std::shared_ptr<std::string> getBuildLog(const StorePath & path)