Auto merge of #106853 - TimNN:undo-remap, r=oli-obk

Heuristically undo path prefix mappings.

Because the compiler produces better diagnostics if it can find the source of (potentially remapped) dependencies.

The new test fails without the other changes in this PR. Let me know if you have better suggestions for the test directory. I moved the existing remapping test to be in the same location as the new one.

Some more context: I'm exploring running UI tests with remapped paths by default in rust-lang/rust#105924 and this was one of the issues discovered.

This may also be useful in the context of rust-lang/rfcs#3127 ("New rustc and Cargo options to allow path sanitisation by default").

Author: bors