Caching for release entries

[mikepurvis]

Currently, colcon-distro only looks at the source entries from a distribution.yaml— this was the primary use case that interested Clearpath, so that's what we built, but to truly be a replacement for rosdistro_build_cache, it would need to be able to maintain a release cache as well.

We won't be working on this, but I'm noting it as an issue/limitation for which outside contributions would be welcome.