specs: remove "or ''" from Spec comparisons

Since `lazy_lexicographic_ordering` handles `None` comparison for us, we
don't need to adjust the spec comparators to return empty strings or
other type-specific empty types. We can just leverage the None-awareness
of `lazy_lexicographic_ordering`.

- [x] remove "or ''" from `_cmp_iter` in `Spec`
- [x] remove setting of `self.namespace` to `''` in `MockPackage`

lib/spack/spack/spec.py

@@ -3617,8 +3617,8 @@ def eq_dag(self, other, deptypes=True, vs=None, vo=None):
 
     def _cmp_node(self):
         """Yield comparable elements of just *this node* and not its deps."""
-        yield self.name or ''
-        yield self.namespace or ''
+        yield self.name
+        yield self.namespace
         yield self.versions
         yield self.variants
         yield self.compiler

lib/spack/spack/util/mock_package.py

@@ -81,8 +81,8 @@ class MockPackageMultiRepo(object):
 
     def __init__(self):
         self.spec_to_pkg = {}
-        self.namespace = ''
-        self.full_namespace = 'spack.pkg.mock'
+        self.namespace = 'mock'                 # repo namespace
+        self.full_namespace = 'spack.pkg.mock'  # python import namespace
 
     def get(self, spec):
         if not isinstance(spec, spack.spec.Spec):
@@ -92,6 +92,10 @@ def get(self, spec):
         return self.spec_to_pkg[spec.name]
 
     def get_pkg_class(self, name):
+        namespace, _, name = name.rpartition(".")
+        if namespace and namespace != self.namespace:
+            raise spack.repo.InvalidNamespaceError(
+                "bad namespace: %s" % self.namespace)
         return self.spec_to_pkg[name]
 
     def exists(self, name):