Fix documentation build issues because of Sphinx limitations

The recent refactoring done in abe741a9, then used in swh-loader-svn@72dfc41,
introduced build issues with the documentation:

    …/swh/loader/svn/directory.py:docstring of swh.loader.svn.directory.SvnDirectoryLoader.snapshot:1:
        WARNING: more than one target found for cross-reference 'Snapshot': swh.fuse.fs.artifact.Snapshot, swh.model.model.Snapshot
    …/swh/loader/svn/directory.py:docstring of swh.loader.svn.directory.SvnDirectoryLoader.cnts:1:
        WARNING: more than one target found for cross-reference 'Content': swh.fuse.fs.artifact.Content, swh.model.from_disk.Content, swh.model.model.Content
    …/swh/loader/svn/directory.py:docstring of swh.loader.svn.directory.SvnDirectoryLoader.dirs:1:
        WARNING: more than one target found for cross-reference 'Directory': swh.fuse.fs.artifact.Directory, swh.model.from_disk.Directory, swh.model.model.Directory

vlorentz explained the cause:

> SvnDirectoryLoader inherits from BaseDirectoryLoader which inherits
> from NodeLoader, which defines:
>
>     self.snapshot: Optional[Snapshot] = None
>
> and it loses the annotation's value (only keeps its string
> representation) because of the inheritence:
> https://github.com/sphinx-doc/sphinx/issues/10124

In order to fix this, we now use a qualified type reference in the
initializers of NodeLoader and BaseDirectoryLoader.