Add `current_module_package_info` macro

[fmeum]

This macro makes it easy for Bazel modules to declare a package_info without having to provide any information manually.

Work towards https://github.com/bazelbuild/bazel/discussions/23166

It's a good idea to open an issue first for discussion.

• [x] Tests pass
• [x] Tests and examples for any new features.
• [x] Appropriate changes to README are included in PR

[fmeum]

@aiuto This is ready for review now that the Rules Authors SIG approved the purl spec: https://github.com/package-url/purl-spec/pull/317

[fmeum]

@aiuto Friendly ping

[fmeum]

@aiuto @shs96c Friendly ping