Allow dictionary input to `BIDSLayout(derivatives=<>)`

[tsalo]

When searching across multiple derivatives datasets, I think it would be useful to be able to include a dataset "name" in layout queries. For example:

from bids.layout import BIDSLayout

BIDSLayout(raw_bids_dir, derivatives={'smriprep': smriprep_dir, 'aslprep': aslprep_dir})
layout.get(dataset='smriprep', suffix='T1w')