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

The check for directory should have always been here. The early warning
test doesn't work for `bst checkout` because caches haven't been configured
at the point of the test, so I've had to remove it.

The return value is always upload_vdir.ref.

Also add docstring to method and remove return value since it was useless

Remote exec: Remove early warning and check directory is not None

See merge request !800