(PUP-1421) Remove flawed source validation in package provider appdmg.

* appdmg's "source" may be a URL that redirects to the actual dmg (and thus not end in .dmg)
* actual download is renamed to "name" anyway
* hdiutil doesn't require the dmg file to have a filename ending in .dmg

More Details: https://tickets.puppetlabs.com/browse/PUP-1421