Source plugin implementations can now specify that they need access to
previously staged sources by specifying
`requires_previous_sources_track` and/or
`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)

`pip` source plugin can stage python packages that are either specified
directly in the element definition or picked up from `requirements.txt`
from previous sources. In order to support the latter use-case
(which is also the primary motivation for this plugin), this plugin
requires access to previous sources and hence is an example of a
Source Transform source.

Also, bump `BST_FORMAT_VERSION` as this patch adds a new core plugin.