Add PythonPackage base class

- Add a PythonPackage class with build system support.
  - Support build phases in PythonPackage
  - Add a custom sanity check for PythonPackages
  - Get rid of nolink dependencies in python packages

- Update spack create to use new PythonPackage class

- Port most of Python packages to new PythonPackage class

- Conducted a massive install and activate of Python packages.
  - Fixed bugs introduced by install and activate.

- Update API docs on PythonPackage

var/spack/repos/builtin/packages/py-setuptools/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PySetuptools(Package):
+class PySetuptools(PythonPackage):
     """A Python utility that aids in the process of downloading, building,
        upgrading, installing, and uninstalling Python packages."""
 
@@ -41,8 +41,3 @@ class PySetuptools(Package):
     version('18.1', 'f72e87f34fbf07f299f6cb46256a0b06')
     version('16.0', '0ace0b96233516fc5f7c857d086aa3ad')
     version('11.3.1', '01f69212e019a2420c1693fb43593930')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix={0}'.format(prefix))