Reorganize resolvers

[luhn]

This PR moves Resolver subclasses and IAssetDescriptor implementations to pyramid.resolver. The remaining contents of pyramid.path gets moved to pyramid._path, so that pyramid.path can expose the same API without a circular dependency.

In a future change, AssetResolver passes IPackageOverrides into PkgResourceAssetDescriptor. This means that AssetResolver now requires registry, which falls back to get_current_registry(). This results in a circular dependency: pyramid.path now imports pyramid.threadlocal imports pyramid.registry imports pyramid.path.

Ways I considered resolving this:

• Make registry a mandatory argument for AssetResolver.
• Make registry optional with no threadlocal fallback. (overrides just won't work)
• Move AssetResolver et al to new package and remove from pyramid.path entirely.
• Move AssetResolver et al to new package and put AssetResolver import at the very bottom of pyramid.path—Almost works but importing pyramid.resolver first still gives circular import.
• Move AssetResolver et al to new package and use pyramid._path to keep AssetResolver in pyramid.path.

The last one (this PR) seems like the only feasible solution, even though _path.py is a bit icky.

[luhn]

@mmerickel would we want to mark pyramid.path.AssetResolver et al as deprecated? I'm hesitant because that's a pretty big deprecation, but if sometime in the future if we want to get from _path.py back to path.py that'll be necessary.