Add 'tag_pattern' feature to git dependencies

[sigurdm]

Re: #1250

With this proposal you can use a git repo as a versioned package repository.

For example write:

dependencies:
  foo:
    git:
      url: https://github.com/foo/foo
      tag_pattern: v{{version}}
    version: ^2.0.0

And all commits in the git repo that are tagged according to v{{version}} will be inspected. If they contain a pubspec.yaml with name: foo and the correct version number they will be considered for resolution.

This feature is explicitly opt-in:

• any git dependency without a tag_pattern retain the existing behavior.
• The depending pubspec need to be at least at version 3.9.0

[jonasfj]

I kind of wish this migration could have been done in steps.

1. Change Description -> ResolvedDescription
2. Move hasMultipleVersions
3. Adopt usage of hasMultipleVersions instead of is HostedSource
4. ..
5. Support for tag-pattern

But I can also see how it would cause a lot of conflicts to do that.