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

[sam-leonard-ct]

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.