Add package_class method to spec.

- Shouldn't call .package from within things like normalize() and
  concretize() beacuse spec may be inconsistent.

- Add `.package_class` property so that we can get at package metadata
  without constructing a Package with a Spec.

- should be faster than `.package` was, anyway. Use where possible.

lib/spack/spack/concretize.py

@@ -238,7 +238,7 @@ def concretize_variants(self, spec):
            the default variants from the package specification.
         """
         changed = False
-        for name, variant in spec.package.variants.items():
+        for name, variant in spec.package_class.variants.items():
             if name not in spec.variants:
                 spec.variants[name] = spack.spec.VariantSpec(name, variant.default)
                 changed = True

lib/spack/spack/repository.py

@@ -316,6 +316,11 @@ def get(self, spec, new=False):
         return self.repo_for_pkg(spec).get(spec)
 
 
+    def get_pkg_class(self, pkg_name):
+        """Find a class for the spec's package and return the class object."""
+        return self.repo_for_pkg(pkg_name).get_pkg_class(pkg_name)
+
+
     @_autospec
     def dump_provenance(self, spec, path):
         """Dump provenance information for a spec to a particular path.
@@ -550,7 +555,7 @@ def get(self, spec, new=False):
 
         key = hash(spec)
         if new or key not in self._instances:
-            package_class = self._get_pkg_class(spec.name)
+            package_class = self.get_pkg_class(spec.name)
             try:
                 copy = spec.copy() # defensive copy.  Package owns its spec.
                 self._instances[key] = package_class(copy)
@@ -715,7 +720,7 @@ def _get_pkg_module(self, pkg_name):
         return self._modules[pkg_name]
 
 
-    def _get_pkg_class(self, pkg_name):
+    def get_pkg_class(self, pkg_name):
         """Get the class for the package out of its module.
 
         First loads (or fetches from cache) a module for the

lib/spack/spack/spec.py

@@ -353,7 +353,7 @@ def constrain(self, other):
     @property
     def concrete(self):
         return self.spec._concrete or all(
-            v in self for v in self.spec.package.variants)
+            v in self for v in self.spec.package_class.variants)
 
 
     def copy(self):
@@ -498,6 +498,14 @@ def package(self):
         return spack.repo.get(self)
 
 
+    @property
+    def package_class(self):
+        """Internal package call gets only the class object for a package.
+           Use this to just get package metadata.
+        """
+        return spack.repo.get_pkg_class(self.name)
+
+
     @property
     def virtual(self):
         """Right now, a spec is virtual if no package exists with its name.
@@ -1161,7 +1169,7 @@ def validate_names(self):
 
             # Ensure that variants all exist.
             for vname, variant in spec.variants.items():
-                if vname not in spec.package.variants:
+                if vname not in spec.package_class.variants:
                     raise UnknownVariantError(spec.name, vname)