Improve error messages for errors during package removal.
Warn when using -m32/-m64 directly.
Add --force-remove switch and reimplement PackageManager.remove.

remove() now first checks for untracked files and leaves the package untouched if it finds some and no force_remove is supplied. This means that after the user gets an error concerning untracked files, it's possible to rerun dub with --force-remove to remove the package cleanly.