Tiny internal code reorganization to ease Agama development

[ancorgs]

agama-project/agama#1448 introduces a new approach to manage storage devices at Agama. Instead of relying on the Y2Storage proposals living in this repository, that PR introduces a new Y2Storage::AgamaProposal class living at the Agama repository.

That new proposal needs to:

• Reuse the logic to detect shadowed volumes
• Ensure the SpaceMaker does not clean up the attribute #forced_ptable_type for all devices just for the sake of it (it doesn't seem to be a clear need for such initialization).

All that could be fixed on the Agama side:

• In the first case it would be relatively easy to just duplicate the code, since it's only 5 lines (including logging).
• In the second case, the corresponding SpaceMaker method can be redefined by the Agama proposal to leave out the excessive initialization.

But this PR offers an alternative by modifying the code directly at the yast2-storage-ng repository, which may be considered cleaner.

[coveralls]

coverage: 97.804%. remained the same
when pulling 6630f78 on ancorgs:agama_shared_classes
into e62c707 on yast:master.

[joseivanlopez]

LGTM

[github-actions]

✅ Autosubmission job #10491232500 successfully finished
✅ Created submit request #1195128