WIP: Add cargo source plugin

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

This is marked WIP because it hard requires BuildStream >= 1.3.1.

Essentially this integrates rust/cargo builds more seamlessly into the BuildStream workflow, and it means we don't ever have to call utils/generate_cargo_dependencies.py again.

This branch:

  • Adds a new cargo source plugin which reads any Cargo.lock file in place in order to automatically determine the dependencies which need to be downloaded
  • Removes the old crate plugin
  • Removes the utils/generate_cargo_dependencies.py which needed to be run manually to update librsvg.bst
  • Makes the librsvg.bst a cargo element

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