Expand the apt provider to support virtual packages

This enables apt and dpkg providers to install virtual packages. There is an attribute named allow_virtual which by default is set to true which enables the functionality to install virtual packages, the same behaviour can be found for rpm provider. If a package that is defined as virtual by default it will search a real package on that system. Example for virtual package named rubygems: package { 'rubygems':     ensure => 'latest' } It will find ruby as the real package and it will be installed instead of rubygems. Note: this also can do unwanted behaviour if someone wants to uninstall a virtual package Example: package {'rubygems': ensure => 'absent' } This by default will remove ruby package. If the user don't want this functionality it can be done by setting the attribute allow_virtual to false. Example: package {'rubygems': ensure => 'absent' allow_virtual => false }