Allow more compact storage of provided checksums

While the repository layout makes sense for storage outside of a project, it would be more convenient to store checksums in a single file (per algorithm) when keeping checksums along when storing these checksums within a project. This makes the storage easier to version control and avoids the overhead of storing a lot of files in version control what often creates some overhead.

Ideally, Maven could support such files out of the box by shipping a provider for such files.