bugfix: fix spack spec --yaml

- repo membership test was broken by the refactor of spack/__init__.py

- refactor singleton so that 'spec in repo' works again for `spack.repo.path`

- fix spec command and add basic tests for `spack spec` and `spack spec --yaml`

lib/spack/llnl/util/lang.py

@@ -558,6 +558,12 @@ def __getattr__(self, name):
     def __getitem__(self, name):
         return self.instance[name]
 
+    def __contains__(self, element):
+        return element in self.instance
+
+    def __iter__(self):
+        return iter(self.instance)
+
     def __str__(self):
         return str(self.instance)