Do not require 'rubygems' from 'rubygems/ext'

RubyGems checks all gems for requirable files it can load matching the
given name.  If a gem requires extension building RubyGems could enter a
state where Gem::Ext was not yet loaded (because rubygems/ext was
partially loaded) which would result in a missing constant exception.

Now RubyGems assumes that module Gem exists when rubygems/ext is loaded.
This avoids the problem of a missing Gem::Ext and allows checking for
requirable files to proceed properly.

Bug discovered by Koichi Sasada.