distributions; this replaces the previous "iter_distributions()" API.
Added basic platform support to Distribution and AvailableDistributions so
that platform-independent distros as well as local platform-compatible
distros are acceptable.  The actual platform scheme is currently delegated
to distutils.util.get_platform(), but needs to be replaced with a better
scheme of some kind, especially for OS X.

--HG--
branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041004