Move test to unit tests.
Non core plugins require an explicit version number.

This is currently implemented at the bintray resolver level as it is the only thing that resolves non core plugins.
Add Supplier general type.

A supplier supplies some object for a managed scope.
Partial implementation of plugin resolution caching.

This is a first pass, and is not quite correct or fully functional. The handling of version numbers is at the wrong level (dependency resolution vs. plugin resolution) and there is no TTL control over the cached values.
Some extra test coverage for remote plugin resolution.
Insert story for allowing plugin authors to test that their plugins work with the new mechanism.