Remove plural-form alias for require_package()

Get rid of require_packages(), keeping only require_package(), and making it
variadic.

(arity -2 means "1 or more arguments"; see
 <http://rdoc.info/github/puppetlabs/puppet/Puppet/Parser/Functions.newfunction#newfunction-class_method>)

Change-Id: I231ad4e5c545a2de563d141d62bbf3b94118a766