Fix #608: broken numpy build.

- Failed to catch all instances of modify_module when it was renamed
  to setup_dependent_package.

- Refactored remaining modify_module calls.

- Also modified Python's setup_dependent_package slightly: only
  creates empty site-packages directory for Python extensions now, not
  for all dependents.

var/spack/repos/builtin/packages/ruby/package.py

@@ -30,7 +30,7 @@ def setup_dependent_environment(self, spack_env, run_env, extension_spec):
         # The actual installation path for this gem
         spack_env.set('GEM_HOME', extension_spec.prefix)
 
-    def modify_module(self, module, spec, ext_spec):
+    def setup_dependent_package(self, module, ext_spec):
         """Called before ruby modules' install() methods.  Sets GEM_HOME
         and GEM_PATH to values appropriate for the package being built.
 
@@ -39,5 +39,5 @@ def modify_module(self, module, spec, ext_spec):
             gem('install', '<gem-name>.gem')
         """
         # Ruby extension builds have global ruby and gem functions
-        module.ruby = Executable(join_path(spec.prefix.bin, 'ruby'))
-        module.gem = Executable(join_path(spec.prefix.bin, 'gem'))
+        module.ruby = Executable(join_path(self.spec.prefix.bin, 'ruby'))
+        module.gem = Executable(join_path(self.spec.prefix.bin, 'gem'))