Removed unused Qdom imports
Use [`feed] Element.t in more places
Cleaned up Versions API

- renamed Versions to Version
- renamed Versions.parsed_version to Version.t.
- format_version -> to_string
- parse_version -> parse
- removed dummy_version
Removed SOLVER_RESULT.version

Instead, just provide ways to sort and display impls. Avoids exposing
dummy_version.