loader.py

Source plugin implementations can now specify that they need access to
previously staged sources by specifying
`BST_REQUIRES_PREVIOUS_SOURCES_TRACK` and/or
`BST_REQUIRES_PREVIOUS_SOURCES_FETCH`, corresponding to access at `track`
and `fetch` times respectively.

Fixes #381.

Replaces !505. For relevant discussion, see this discussion:
!505 (comment 83780747)