WIP: Refactor funcitonality in _ostree.py into ostree.py

WIP because there is still a breaking bug with this refactor (details on !10 (comment 169343625))

The functionality in _ostree.py was originally separate from the ostree plugin because buildstream used is for an ostree backed cache. There is no longer any reason to have this in a separate file.

Proposed changes:

• Move functionality provided by _ostree.py into the ostree plugin.

This is most easily reviewed as individual commits.

This was originally discussed with @juergbi in !1 (comment 166870376)

Closes #7 (closed)