checkout: Add API to directly checkout composefs

checkout: Add API to directly checkout composefs

We were missing the simple, obvious API and CLI to go
from ostree commit -> composefs.

Internally, we had `ostree_repo_checkout_composefs`
with the right "shape" mostly, except it had more code
in the deploy path to turn that into a composefs.

Add a straightforward public API that does what
the deploy code did before, and then the old
API becomes an explicitly internal helper with an `_`
prefix.

Goals:

- Lead towards a composefs-oriented future
- This makes the composefs logic more testable directly

Signed-off-by: Colin Walters <walters@verbum.org>