feat(tvix/glue/derivationStrict): support __structuredAttrs

This adds support to handle the __structuredAttrs argument, which can be
passed to builtins.derivationStrict.

If __structuredAttrs is passed, and set to true, most of the arguments
passed to builtins.derivationStrict are not simply coerced to a string
and passed down to "environments", but instead kept in a more structured
fashion.

Inside ATerm, which is what's relevant as far as path calculation is
concerned, a virtual `__json` environment variable is present,
containing these structured values.

Inside Builds, these structured values are not made available as an
environment variable, but a JSON file (and source-able bash script).

This will need to be respected once we start emitting BuildRequests,
and for that we can probably just parse the `__json` key in
Derivation.environment again - or keep this additionally in
non-serialized form around during Evaluation.
No matter what, this is left for a followup CL.

The existing handle_derivation_parameters and populate_outputs helper
function were removed, as __structuredAttrs causes quite a change
in behaviour, and so handling both in the same place makes it more
readable.

There's some open questions w.r.t. string contexts for structured attrs
itself. A TODO is left for this, but at least path calculation for
individual structured attrs derivations are correct now.

Part of b/366.

Change-Id: Ic293822266ced6f8c4826d8ef0d2e098a4adccaa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10604
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/glue/src/tvix_build.rs

@@ -87,7 +87,8 @@ where
     );
 
     handle_pass_as_file(&mut environment_vars, &mut additional_files)?;
-    // TODO: handle structuredAttrs.
+
+    // TODO: handle __json (structured attrs, provide JSON file and source-able bash script)
 
     // Produce inputs. As we refer to the contents here, not just plain store path strings,
     // we need to perform lookups.