Lets MODULE.bazel point starlark_repository at a workspace-local path
instead of an http_archive. Useful when the source already lives on
disk — e.g. a git submodule — and we want to skip the network fetch
entirely. The bcr-frontend project hits this with ~449 overlay-bzl
modules that all derive from the same bazel-central-registry archive;
emitting one `.archive(urls=...)` per module trips GitHub's 429 rate
limit on cold caches.

Changes
- extensions/starlark_repository.bzl: new `local` tag class with attrs
  {name, path, build_directives, build_file_generation, languages,
  cfgs, imports, imports_out (default "imports.csv"), deleted_files,
  reresolve_known_proto_imports, importpath}. The extension impl maps
  user-facing `path` to the underlying `local_path` attr and dispatches
  to the same starlark_repository repo rule used by `.archive`.
- rules/proto/proto_repository.bzl: in `_proto_repository_impl`'s
  local_path branch, resolve workspace-relative paths against the main
  workspace root (via Label("@@//:MODULE.bazel").dirname) before
  passing them to ctx.watch_tree() and fetch_repo's --path arg. Bare
  strings were otherwise interpreted relative to the external repo's
  own dir, where the directory doesn't exist.
- rules/proto/proto_repository.bzl: in _generate_proto_repository_info,
  return empty string when imports_out is unset, so a future caller
  with no imports_out can't trip `exports_files([""])`.

Usage:

    starlark_repository = use_extension(
        "@build_stack_rules_proto//extensions:starlark_repository.bzl",
        "starlark_repository",
    )
    starlark_repository.local(
        name = "bzl.colordiff---1.0.22",
        path = "data/bazel-central-registry/modules/colordiff/1.0.22/overlay",
        build_directives = ["gazelle:starlarkrepository_root"],
        build_file_generation = "clean",
        languages = ["starlarkrepository"],
    )
    use_repo(starlark_repository, "bzl.colordiff---1.0.22")