[Distutils] EntryPoint.require() when dist=None

[Ashley Walsh]

EntryPoint.require() fails when extras==() and dist==None because it  
still attempt to call dist.requires().  Should the method be changed  
to somthting like::

     def require(self, env=None, installer=None):
             if self.extras and not self.dist:
                 raise UnknownExtra("Can't require() without a  
distribution", self)
             if self.dist:
	            map(working_set.add,
         	        working_set.resolve(self.dist.requires 
(self.extras),env,installer))

Cheers,
Ashley