Fixes 5167
When ``a.rs`` and ``a/mod.rs`` are both present we would emit an error
message telling the user that the module couldn't be found. However,
this behavior is misleading because we're dealing with an ambiguous
module path, not a "file not found" error.
Is the file ``a.rs`` or is it ``a/mod.rs``? Rustfmt can't decide, and
the user needs to resolve this ambiguity themselves.
Now, the error message displayed to the user is in line with what they
would see if they went to compile their code with these conflicting
module names.
