Fix spack package inheritance for module variables (#10097)

* we weren't properly setting module variables for the root package in a DAG -- just for transitive dependencies.

var/spack/repos/builtin.mock/packages/multimodule-inheritance/package.py

@@ -0,0 +1,20 @@
+# Copyright 2013-2018 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import spack.pkg.builtin.mock.simple_inheritance as si
+
+
+class MultimoduleInheritance(si.BaseWithDirectives):
+    """Simple package which inherits a method and several directives"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/multimodule-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    depends_on('openblas', when='+openblas')
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/simple-inheritance/package.py

@@ -13,6 +13,12 @@ class BaseWithDirectives(Package):
     variant('openblas', description='Activates openblas', default=True)
     provides('service1')
 
+    def use_module_variable(self):
+        """Must be called in build environment. Allows us to test parent class
+         using module variables set up by build_environment."""
+        env['TEST_MODULE_VAR'] = 'test_module_variable'
+        return env['TEST_MODULE_VAR']
+
 
 class SimpleInheritance(BaseWithDirectives):
     """Simple package which acts as a build dependency"""