phase-shelve: Enable phase-based object shelves

This series of patches is attempting to solve the problem that even when phase-based shelving is available (internal phase requirement is enabled), the shelving behavior still relies on the .patch and .hg files to track the presence and metadata of shelves. With these changes, the presence of those files is unnecessary for basic operations (shelve, unshelve, --list, --list --patch).

These changes open the door to a client enabling internal phases and discarding or disregarding the .hg and .patch files. This change also opens up opportunities to potentially deprecate patch-based shelves (if internal-phase becomes enabled by default) and thus unify the behavior.