Add gazelle plugin to BCR

[shs96c]

With #400 the gazelle plugin has now been added to the distribution, and it should be possible to ship it to BCR.

[tetromino]

After #400, I don't understand why the gazelle plugin belongs in this git repo:

• skylib and the gazelle plugin now have separate bazel workspaces
• gazelle plugin doesn't get built by bazel build //...
• gazelle plugin doesn't get tested by bazel test //...
• gazelle plugin depends only on skylib's public api
• skylib does not depend on the gazelle plugin at all

But in any case, regardless of where the code lives, before making it releasable, we would need to add distro rules to generate the tarball. Equivalent of what we have already in distribution/BUILD for skylib.

[tetromino]

Distro rules to generate tarball will be added by #424

[tetromino]

bazelbuild/bazel-central-registry#403 merged 🎉