Use attributes of *PackageInfo objects instead of untyped dicts.

This commit does the following:

• Move artifact_identity to BasePackageInfo, which uses a class attribute (and is overriden for ArchivePackageInfo, which needs a custom behavior to override keys). Also moved/improved its test
• Add attributes to *PackageInfo classes, that can be accessed instead of the raw metadata.
• Add a from_metadata class method to all *PackageInfo classes, to parse the raw metadata and build the object from it.
• Pass the PackageInfo object to resolve_revision_from and build_revision instead of untyped dicts.