package_prefs: move class-level cache to PackagePref instance

`PackagePrefs` has had a class-level cache of data from `packages.yaml` for
a long time, but it complicates testing and leads to subtle errors,
especially now that we frequently manipulate custom config scopes and
environments.

Moving the cache to instance-level doesn't slow down concretization or
the test suite, and it just caches for the life of a `PackagePrefs`
instance (i.e., for a single cocncretization) so we don't need to worry
about global state anymore.

- [x] Remove class-level caches from `PackagePrefs`
- [x] Add a cached _spec_order object on each `PackagePrefs` instance
- [x] Remove all calls to `PackagePrefs.clear_caches()`

lib/spack/llnl/util/lang.py

@@ -19,12 +19,6 @@
 ignore_modules = [r'^\.#', '~$']
 
 
-class classproperty(property):
-    """classproperty decorator: like property but for classmethods."""
-    def __get__(self, cls, owner):
-        return self.fget.__get__(None, owner)()
-
-
 def index_by(objects, *funcs):
     """Create a hierarchy of dictionaries by splitting the supplied
        set of objects on unique values of the supplied functions.