Sometimes, pkgutil will return actual files (usually .nib files) as if they were part of the directory.  Microsoft Office is an example of this: in a recent update the file `/Library/Application Support/Microsoft/MAU2.0/Microsoft AutoUpdate.app/Contents/SharedSupport/Microsoft Error Reporting.app/Contents/Resources/en.lproj/MainWindowAlt.nib` was returning from `/usr/sbin/pkgutil --only-dirs --files com.microsoft.package.component` even though it should have been a file instead of a directory.  This caused the `rmdir` command to fail.

This patch will check if we are trying to delete a “directory” that is really a “file” - and if we are, we just delete the file instead.  This will allow packages that get in this state to be uninstalled.  A unit test which can be run using `brew cask-tests` is also included.