Fix equality checking for PathLocations

The current implementation of equality checking for `PathLocation` implicitly relied on `Dir.pwd` because it called `expand_path` without a relative point.

This PR forces consistency of `PathLocation` paths.

The `relative_path` method had been refactored to just return the path relative to the Berksfile. It is used when writting the lockfile and comparing locations.

The new `expanded_path` method is the fullly expanded path on disk, relative to the Berksfile. It is used to actually locate the cookbook on disk.