Implement Gradle metadata marker in published pom/ivy files This commit implements a performance optimization for Gradle metadata. Given that today there's no library published in any repository with Gradle metadata, it's much more likely to find a POM (or Ivy) metadata file for an external dependency, rather than a Gradle metadata file.
If we decided to add `gradleMetadata()` sources by default to all repositories, then we would probably introduce a performance regression to a lot of builds, because we would first try to get Gradle metadata, then fail, and search for POM/Ivy files.
To avoid this, whenever a library is going to be published with Gradle metadata, we will introduce a _marker_ in the published POM or Ivy file. When Gradle _resolves_ such a dependency, it will parse the POM file and look for the marker. If it finds it, then it will _redirect_ to use Gradle metadata instead. This means that when Gradle metadata is present, we will pay the price of looking for a POM or Ivy file first, start parsing, only to discover we should parse Gradle metadata. This should be ok in the beginning, knowing that if `gradleMetadata()` is added, then we would systematically look at Gradle metadata first.
This means that this is a _temporary_ solution, until Gradle metadata becomes widespread. So "temporary" should be understood as several months, if not years.
The marker introduced in POM and Ivy files is _neutral_ for both Ivy and Maven. By this we mean that it uses an XML comment. While not super nice, we couldn't use a custom namespace because both Ivy and Maven fail when parsing this. Properties were considered too, but Ivy doesn't support them so for consistency both models use comments.
It's worth noting that we will still _completely parse_ the POM or Ivy descriptor. It's a waste of time, but it helps in case we find a marker but that for some reason the Gradle metadata file is absent. In this case we fallback on the model we found.
This change also introduces a change in the semantics of the incubating metadata sources API: those should be considered _hints_, and not strong statements anymore.
Finally, should a producer want to avoid publishing Gradle metadata, it's now possible to disable the task that creates the metadata file.