fix(Dependency) ignore extra `version` key with path and git deps (dart-lang/pubspec_parse#14)

Streamline the handling of all dependencies
Improve errors when unsupported keys are provided in dependencies
Better handling (and testing) for invalid `sdk:` values

Handle scp-like URIs for git dependencies
 - See https://git-scm.com/docs/git-clone#_git_urls_a_id_urls_a

Fixes https://github.com/dart-lang/pubspec_parse/issues/13

Prepare for release