Directory and inheritance structure of storage backends

[GeigerJ2]

As noted with @rabbull, all ORM entities for the SQLite backends are contained in storage/sqlite_zip/orm.py, while the other SQLite-based storage backends then import from there, e.g.:

aiida-core/src/aiida/storage/sqlite_dos/backend.py

Line 33 in dd866ce

from aiida.storage.sqlite_zip import models, orm

While it's not critical, functionality-wise, we could still consider a refactor, as it's a bit confusing that everything sqlite ORM related is contained under sqlite_zip, while sqlite_dos and sqlite_temp are actually implemented in directories at the same level, so one would expect those to stand on equal footing.

Similarly, I wouldn't actually have expected the SqliteDosStorage to inherit from PsqlDosBackend, but rather both of them to have a common parent class, or using composition? Wondering if there was a reason for this structure when the SqliteDosStorage was implemented in #6148. I guess this is mainly historical? Pinging also @unkcpz preaching "composition over inheritance" :D

[unkcpz]

I think it is also a great place to use python protocol for duck typing. If there have N database backend and M file-repository backend, we don't want to end up with NxM storage backend.
We can have a discuss on coding week and see how to initialize the work on it.