This normalizes the path so that we always have the mapping in normal
form. This fixes a bug in the cross-module import tracing, allowing us
to finally enable the test on Windows.
Windows paths attack again. Some paths from the dependency tracker have
mixed folder separators, so comparing textually fail. Additionally some
paths seems to be extended paths (\\?\) which seems not to be liked by
LLVM path utilities.
✔ More informative error messages in case of crashes.
✔ Handling and documenting different cases.
✔ Test cases for different cases.
✔ Make SDKDependencies.swift pass again.
