Backport ability to see previous sources in fetch() and track()

Tristan Van Berkomrequested to merge
tristan/backport-previous-sources into bst-1

This backports:

  • The original work codenamed "source transform" (!568 (merged))
  • The original pip source plugin which leverages the ability to view previously staged sources (also from !568 (merged))
  • The related fix for #1010 (closed) which was introduced by the previous patches (in !1330 (merged))

This will be especially important for implementing a similar plugin to the pip plugin but for cargo/rust projects.

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