Fix  rdmd --makedep(end|file) (issues 12351 and 12354)

This pull request basically makes `rdmd --makedep(end|file)` useful. I don't know if anybody used it before (checking it does work), because I have no idea how it could have worked.

Basically this fixes 2 important issues (use the binary produced as the target instead of the source file, and list all the dependencies as empty targets to avoid make errors if you remove a file). Also the output format is improved to make it both more testeable and human-readable.