Bug: "missing strict dependencies" using import public and go_proto_library

Given :

* three `go_proto_library()` targets **foo**, **bar**, and **baz**
* **foo** depends on **bar**, and **bar** depends on **baz**
* **bar** uses [import public "baz.proto"](https://protobuf.dev/programming-guides/proto3/#importing)

The `protoc-gen-go` tool generates code for **foo** (`foo.pb.go`) that looks like:

```golang
// Code generated by protoc-gen-go. DO NOT EDIT.
// versions:
// protoc-gen-go v1.36.3
// protoc v5.27.0
// [...]

import (
baz "[...]/baz_go_proto"
_ "[...]/bar_go_proto"
```

The `compilepkg` strict dependencies check scans this file for imports, sees the import of **baz**, and then errors because **foo** did not declare a direct dependency on **baz**.

https://github.com/bazel-contrib/rules_go/blob/393faea31d43012fbb7887dfbddb869cb5cf58f5/go/tools/builders/importcfg.go#L100

However, I think this behavior is a bug. If I understand correctly, `import public` is supposed to enable moving messages without breaking downstream consumers. The alternative is to update all users atomically, but it can be difficult to impossible to do that in:

* a large and active monorepo, or
* an SDK where you've promised your customers backwards compatibility

## Steps to reproduce

Clone my fork

```
git clone -b sloretz_repro_import_public_issue https://github.com/sloretz/rules_go.git
```

Run this test

```
bazel test //tests/core/go_proto_library_import_public:import_public_test
```

Observe this failure:

```
compilepkg: missing strict dependencies:
/home/sloretz/.cache/bazel/_bazel_sloretz/e4e5127b5b9f1e33f9a706fce20cac1d/sandbox/linux-sandbox/4/execroot/io_bazel_rules_go/bazel-out/k8-fastbuild/bin/tests/core/go_proto_library_import_public/dog_go_proto_/github.com/bazelbuild/rules_go/tests/core/go_proto_library_import_public/dog_go_proto/dog.pb.go: import of "github.com/bazelbuild/rules_go/tests/core/go_proto_library_import_public/new_dir/dog_collar_go_proto"
No dependencies were provided.
Check that imports in Go sources match importpath attributes in deps.
```

## Suggested fix

The `proto_library()` rule has [an `exports` attribute](https://bazel.build/reference/be/protocol-buffer#proto_library) for the `import public` use case. How about `go_proto_library()` gets an `exports` attribute as well? Then **bar** could declare an export of **baz**, and **foo** can be ignorant of protos being moved upstream of it.

The `exports` attribute would need to set some provider such that **foo**'s call to `compilepkg` would get a new [-arc argument for **baz**](https://github.com/bazel-contrib/rules_go/blob/393faea31d43012fbb7887dfbddb869cb5cf58f5/go/tools/builders/compilepkg.go#L52)

Start with tests/core/go_proto_library_import_public:import_public_test and reproduce the failure described in the issue. Read go/tools/builders/importcfg.go around the strict-dependencies scan and go/tools/builders/compilepkg.go around the arc argument, then inspect how proto_library exports are represented. Done means the test passes for the import public scenario without requiring foo to declare baz directly.

Written by the indexing model from the issue text.