Detect existing overlaybd layers during conversion

[adam-rl]

These layer.tar files (with overlaybd.commit inside) should be used directly and not applied as a file.

What this PR does / why we need it: Prevents the incorrect conversion of an existing overlaybd commit file. With this change, base layers that do not need to be converted have the same digest as their originals.

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?