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

tests/structured-attrs-shell.nix

@@ -0,0 +1,19 @@
+with import ./config.nix;
+let
+  dep = mkDerivation {
+    name = "dep";
+    buildCommand = ''
+      mkdir $out; echo bla > $out/bla
+    '';
+  };
+in
+mkDerivation {
+  name = "structured2";
+  __structuredAttrs = true;
+  outputs = [ "out" "dev" ];
+  my.list = [ "a" "b" "c" ];
+  exportReferencesGraph.refs = [ dep ];
+  buildCommand = ''
+    touch ''${outputs[out]}; touch ''${outputs[dev]}
+  '';
+}

tests/structured-attrs.sh

@@ -8,3 +8,9 @@ nix-build structured-attrs.nix -A all -o $TEST_ROOT/result
 
 [[ $(cat $TEST_ROOT/result/foo) = bar ]]
 [[ $(cat $TEST_ROOT/result-dev/foo) = foo ]]
+
+export NIX_BUILD_SHELL=$SHELL
+[[ ! -e '.attrs.json' ]]
+env NIX_PATH=nixpkgs=shell.nix nix-shell structured-attrs-shell.nix \
+    --run 'test -e .attrs.json; test "3" = "$(jq ".my.list|length" < .attrs.json)"'
+[[ ! -e '.attrs.json' ]]