Feature request: "metadata-only" publishing

Now that the Gradle Plugin Portal proxies [MavenCentral](https://blog.gradle.org/portal-jcenter-impact), would it be possible to add a "metadata-only" mode to publishing plugins?

It would allow to get the SEO and discoverability of the Gradle Plugin Portal while consolidating the download stats in Maven Central. Since Maven Central is already required to host the plugin transitive dependencies, might as well go all way in and host everything there.

In addition to consolidated stats, I'm hoping this can also simplify the maintenance of plugins: only one repo to monitor for exploits/security, no need to tweak the `exclusiveContents {}` blocks in `repositories`, no risk of discrepencies, [support for Maven Relocations](https://maven.apache.org/guides/mini/guide-relocation.html), etc...

Thoughts?

Start by reviewing the Gradle Plugin Portal proxy context and the linked Maven Central relocation guidance. The issue does not name implementation files, tests, or an entry point; clarify the metadata-only publishing behavior and acceptance criteria before identifying the affected publishing plugins and tests.

Written by the indexing model from the issue text.