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-genders/package.py

@@ -36,6 +36,9 @@ class PyGenders(Package):
             url='https://github.com/chaos/genders/releases/download/genders-1-22-1/genders-1.22.tar.gz')
     extends('python')
 
+    # FIXME: Missing a dependency on genders
+    # #include <genders.h>
+
     def install(self, spec, prefix):
         configure("--prefix=%s" % prefix)
         make(parallel=False)