if all requirements can be met with items already in the working set.
Don't eagerly determine whether a path is a directory.  Avoid redundant
path operations, etc.  These changes dropped the test suite runtime from
over 3.4 seconds to around .34 seconds.

--HG--
branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041176