Improve `Sources` metadata

[effigies]

BIDS Derivatives files SHOULD have a list of URIs to their sources. These should always be findable files, either in an input dataset (including templateflow) or in the current output dataset.

Files in the current dataset are prefixed bids::. Files in the input dataset should be prefixed bids:raw:. Files in templateflow should be prefixed bids:templateflow:. Files in a precomputed derivative dataset passed as --derivatives myds=$PATH should be prefixed bids:myds:.