- Previously, dependencies and dependency_types were stored as separate
dicts on Package.
- This means a package can only depend on another in one specific way,
which is usually but not always true.
- Prior code unioned dependency types statically across dependencies on
the same package.
- New code stores dependency relationships as their own object, with a
spec constraint and a set of dependency types per relationship.
- Dependency types are now more precise
- There is now room to add more information to dependency relationships.
- New Dependency class lives in dependency.py, along with deptype
definitions that used to live in spack.spec.
Move deptype definitions to spack.dependency
0e8bb9ec5e