Merge pull request #783 from stevvooe/next-generation

Correct references after move of Manifest into storage package
Clean up layer storage layout

Previously, discussions were still ongoing about different storage layouts that
could support various access models. This changeset removes a layer of
indirection that was in place due to earlier designs. Effectively, this both
associates a layer with a named repository and ensures that content cannot be
accessed across repositories. It also moves to rely on tarsum as a true
content-addressable identifier, removing a layer of indirection during blob
resolution.
Merge pull request #782 from stevvooe/cleanup-storage-mapping

Clean up layer storage layout