Provide an alternative without redirects

As a follow-up to
* https://github.com/gradle/gradle/issues/15406

In some contexts, it makes more sense to use the Plugin portal only for the artifacts published there and have no redirect to the Maven Central.

We should consider providing an alternative entry point for these scenarios.
Note that the current behavior will be preserved by default for compatibility. One important aspect here is plugin dependencies that are usually on Maven Central: without redirects, they won't be found. So Maven Central repository should be declared along with the Plugin Portal Repository to use them.

We might consider this option in future versions of Gradle.

Start by reviewing the linked issue 15406 and the current Plugin Portal redirect behavior described here. Define the alternative entry point and its compatibility behavior, including how plugin dependencies from Maven Central are resolved. Done means the proposed no-redirect path is specified without changing the default behavior.

Written by the indexing model from the issue text.