Stable C++ library publishing layout in a Maven repository

Once teams start to publish C++ libraries to Maven repositories, we should make the layout stable:

- Newer versions of Gradle be able to consume libraries that have already been published.
- Libraries published by new versions of Gradle should be able to be consumed by older versions of Gradle.
- Enable use of the module metadata file during resolution by default.

Here the "layout" means the formats, attributes, naming schemes, packaging, etc of a C++ library in a Maven repository. This doesn't any other aspects of building or publishing or consuming C++ libraries, such as APIs or DSLs. Making these other things stable are not in scope for this story and are instead part of the larger epic.

Start by reviewing the Maven repository layout and Gradle module metadata requirements described in the issue. Define the formats, attributes, naming schemes, and packaging that must remain compatible across Gradle versions, while excluding APIs, DSLs, and other out-of-scope concerns. Done means the stable layout and compatibility expectations are specified and verified.

Written by the indexing model from the issue text.