Versioning rework

[waj]

This ended up being a not so small PR and I have more changes coming, but I prefer to split here now that at least the tests are passing 😅

This is mostly a refactor, although I fixed some cases of locks with commits that were either not working or working not totally on purpose.

For the user, there are two (non breaking) visible changes:

• There is a shard_name.version file for each shard within the lib directory. This replaces the .sha1 file that was generated for shards not coming from a release version.
• The shard.lock file now contains version for both releases and git refs. For versions it's the same a before, but for git refs the version is stores something like: 0.9.0+git.commit.ed686ad3015f48f8b22cf4fbec80e8c8088ef44b

Internally there are some refactors:

• Versions contains metadata to represent references to a commit. No more need to check either commit or version.
• The interface of the Resolver class changed slightly:
  • versions_for(Dependency) now returns the list of versions (releases or refs) matching the dependency requirement.
  • spec(version) works only with version strings, no more refs. (refs are transformed into versions first by calling the previous method)
  • install is performed by version also, no more refs.
• Lock files with old format can still be read and they will be transformed when needed.
• MolinilloSolver is now pretty much agnostic to git refs. There is still a minor reference to git metadata that I plan to remove soon. Understanding of whether a version matches a requirement is delegated to the resolver.

All these changes (and the ones coming) will make it easier soon to add or fix features:

• Know if certain commit still matches a ref
• Provide information about outdated refs (and fix Index out of bounds error in shards outdated #325)
• Install from locks without running the resolver
• Better support for overrides of shards (i.e: a sub-dependency with a version with an explicit dependency in shard.yml)