Write invalid remapped paths to unit files

[kastiglione]

When using FileManager::getFile(), if the path does not exist, then the unit file will contain a blank path for that field. Using getVirtualFile() will ensure that even invalid paths are written into the unit file. Invalid paths are useful for two reasons:

1. The file may not exist now but is expected to exist later
2. Debugging path remapping in the output of index-import (via absolute-unit)

In the second case, it's better to see an invalid path than an empty path.

The solution should be to use getVirtualFile() everywhere, but this function asserts if called with an empty path (which can legitimately happen[1]). The final solution is to call getVirtualFile in all cases except for the empty string which requires getFile.

1. For example, .pcm files have an empty MainFilePath