Add annotation support for transient crates.

[matts1]

No description provided.

[matts1]

@illicitonion I've done a complete rewrite of annotations according to your suggestions of inlining them into MODULE.bazel.

[gferon]

I tested this PR on our current setup (still trying to move to bzlmod) and it fails on json.encode when supplying labels like build_script_tools = ["@com_google_protobuf//:protoc"] with the following error:

(11:07:32) ERROR: Traceback (most recent call last):
        File "/Users/gferon/.cache/bazel/8d38801da57cbc2fb5fa58e853c562bd/external/rules_rust~override/crate_universe/extension.bzl", line 182, column 58, in _crate_impl
                annotation = _crate_universe_crate.annotation(**{
        File "/Users/gferon/.cache/bazel/8d38801da57cbc2fb5fa58e853c562bd/external/rules_rust~override/crate_universe/private/crate.bzl", line 179, column 23, in _annotation
                return json.encode((
Error in encode: at tuple index 1: in struct field .build_script_tools: at list index 0: cannot encode Label as JSON

I'm not sure yet what the right fix would be though.

EDIT: I tried to "stringify" those labels, but the produced JSON can't be parsed anymore by cargo-bazel, so I'm definitely doing something wrong here.

[illicitonion]

I tested this PR on our current setup (still trying to move to bzlmod) and it fails on json.encode when supplying labels like build_script_tools = ["@com_google_protobuf//:protoc"] with the following error:

(11:07:32) ERROR: Traceback (most recent call last):
        File "/Users/gferon/.cache/bazel/8d38801da57cbc2fb5fa58e853c562bd/external/rules_rust~override/crate_universe/extension.bzl", line 182, column 58, in _crate_impl
                annotation = _crate_universe_crate.annotation(**{
        File "/Users/gferon/.cache/bazel/8d38801da57cbc2fb5fa58e853c562bd/external/rules_rust~override/crate_universe/private/crate.bzl", line 179, column 23, in _annotation
                return json.encode((
Error in encode: at tuple index 1: in struct field .build_script_tools: at list index 0: cannot encode Label as JSON

I'm not sure yet what the right fix would be though.

EDIT: I tried to "stringify" those labels, but the produced JSON can't be parsed anymore by cargo-bazel, so I'm definitely doing something wrong here.

#2394 should fix this

[illicitonion]

This LGTM, thanks! Just one comment around booleans, then I think we're good to go.

[matts1]

I've just rebased. There was also apparently some problems with using relative labels, as they were being evaluated relative to the wrong package. I've fixed it up now by turning them into strings.

[illicitonion]

Thanks so much!