Add cargo source plugin (second try)

Closed Tristan Van Berkom requested to merge tristan/cargo-source-plugin-2 into master

This is like !347 (closed), except that it adds cargo to bst_external and loads bst_external as a submodule.

This requires first landing in bst_external (and then this branch needs to be updated to point to a new ref of the bst_external submodule).

This fixes #136 (closed)

Edited by Tristan Van Berkom

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