This fixes an issue where the re-instantiated Source used with
Source mirroring enabled is not completely initialized.

Failing to load the ref from the project.refs file for instance,
will result in a crash at `fetch` time.

This fixes issue #666

source.py: Fix re-instantiation

Closes #666

See merge request !817

This is benign because a _yaml.ChainMap doesn't raise a
KeyError, but it's not required to be a ChainMap.

Note that the ProvenanceFile's names will be incorrect after loading
from the cache, but this is currently only used for writeback, which
isn't used in junctions.