[Distutils] More DWIMmy goodness for easy_install

[Phillip J. Eby]

FYI, I just checked in some additional target directory checks for 
easy_install, based on an idea inspired by a comment from Exarkun on my 
blog.  It seems that a lot of the most recent "new" problems reported with 
setuptools have to do with site-packages directories under secondary 
prefixes, like Debian's /usr/local or various 64-bit distros' 
/usr/lib64.  EasyInstall hasn't previously been able to detect that these 
are valid "site" directories, leading to unnecessary pain for users of 
these platforms.

So, the latest SVN version now contains logic to verify the "site"-ishness 
of the installation directory, via the crude (but effective) expedient of 
writing a .pth file to the directory and spawning another Python process to 
see if the .pth file actually gets processed.  This should almost 
completely eliminate the need for --site-dirs, and along with it, the need 
to actually read the installation instructions.  :)

In fact, the only time now that somebody should care about the "custom 
installation location" docs is if they want to set up a custom "site" 
directory and *not* use PYTHONPATH to do it.

In addition to this new empirical test for site-ness, I've also added 
writability and existence checks, with hopefully-useful error messages.  So 
we should now be a lot closer to a place where package developers can feel 
safe using ez_setup as part of their installation process, even if they're 
very particular about the user experience on a wide variety of platforms.

However, testing is appreciated, as always.  I'm particularly interested in 
whether this version can correctly deal with installation targets like 
/usr/lib64 and Debian's /usr/local/python2.x/site-packages directories, 
*without* needing to specify site-dirs on the command line or in a config file.