Inverse of Source.translate_url for more user friendly bst source track

While using the cargo2 plugin to track a git dependency I came across an issue where I had to manually fix the tracked Cargo.lock contents to change the URLs to aliased ones.
IMO having a method that takes an unaliased URL and returns one aliased with the most specific alias from project.conf would be incredibly helpful for implementing something like this as currently it is not possible to implement this without accessing private parts of the Source class from a plugin.

Start by reading Source.translate_url and the project.conf alias handling described in the issue, then inspect how the cargo2 plugin currently tracks git dependencies and accesses Source internals. Done means an exposed method accepts an unaliased URL and returns the URL using the most specific configured alias, allowing Cargo.lock tracking without private Source access.

Written by the indexing model from the issue text.