Ensure mainSourceFile is deduplicated properly. Fixes #1407.
Ensure mainSourceFile is deduplicated properly. Fixes #1407.

This forces the mainSourceFile to be normalized and converted to the native path format before being added to sourceFiles, so that when adding files later on with collectFiles matches up properly.

gedaiu/dub,D-Programming-Language/dub,Geod24/dub,rjframe/dub