Unify (remote) path handling in backend

[fm3]

We now use URI in some places, in others Path and in others yet String.
This goes through RemoteSourceDescriptors and DataVaults.

Different codepaths exist to perform checks on the paths and to normalize them.

URI getScheme may return null, which needs to be checked and isn’t caught by the type system.

We should make an overview over the different cases and consider if a unified solution can be found.