Apply repo mapping to `cc_shared_library`'s `exports_filter`

[fmeum]

This is required for exports_filter to match any external repositories with Bzlmod enabled.

native.package_relative_label is used to convert the user-specified filter strings, which are always valid labels, to canonical label literals that are then passed to the actual cc_shared_library rule.

Fixes #21872

[fmeum]

Original context is https://bazelbuild.slack.com/archives/CGA9QFQ8H/p1709829043052719

[fmeum]

Switching to draft as I will update it after https://github.com/bazelbuild/bazel/pull/21740 has been merged.

[fmeum]

@bazel-io fork 7.2.0