Move the ReleaseSource trait, GithubEndpoints and ForgejoEndpoints under
src/source/, each in its own file (release_source.rs, github.rs,
forgejo.rs). The submodules are private; mod.rs re-exports their types and
holds for_package, so the rest of the crate imports from crate::source and
never names a host's file. checker.rs keeps only version_from_tag.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
