Use key sorting instead of cmp()

- Get rid of pkgsort() usage for preferred variants.
- Concretization is now entirely based on key-based sorting.
- Remove PreferredPackages class and various spec cmp() methods.
- Replace with PackagePrefs class that implements a key function for
  sorting according to packages.yaml.
- Clear package pref caches on config test.
- Explicit compare methods instead of total_ordering in Version.
- Our total_ordering backport wasn't making Python 3 happy for some
  reason.
- Python 3's functools.total_ordering and spelling the operators out
  fixes the problem.
- Fix unicode issues with spec hashes, json, & YAML
- Try to use str everywhere and avoid unicode objects in python 2.

lib/spack/external/functools_backport.py

@@ -28,3 +28,20 @@ def total_ordering(cls):
             opfunc.__doc__ = getattr(int, opname).__doc__
             setattr(cls, opname, opfunc)
     return cls
+
+
+@total_ordering
+class reverse_order(object):
+    """Helper for creating key functions.
+
+       This is a wrapper that inverts the sense of the natural
+       comparisons on the object.
+    """
+    def __init__(self, value):
+        self.value = value
+
+    def __eq__(self, other):
+        return other.value == self.value
+
+    def __lt__(self, other):
+        return other.value < self.value