Using @import syntax on objc_library

## Description of the problem / feature request:
Title

### Feature requests: what underlying problem are you trying to solve with this feature?
We have lots of Objc modules that also importing Swift codes. #6243 actually solves this issue but the progress went into limbo, so we are facing a quite big blocker right now.

Is there any relevant discussion regarding this? I can fork Bazel to support this, but this would be quite painful from maintenance perspective in the future if not officially supported.

### Have you found anything relevant by searching the web?
https://github.com/bazelbuild/bazel/pull/6243

Start by reading the stalled pull request #6243 and comparing it with the objc_library behavior described here. Done means Bazel officially supports @import syntax for Objective-C modules that import Swift code, with the expected build behavior covered by the relevant tests identified during that investigation.

Written by the indexing model from the issue text.