spack.repo.get() can only be called on concrete specs (#31411)

The goal of this PR is to make clearer where we need a package object in Spack as opposed to a package class.

We currently instantiate a lot of package objects when we could make do with a class.  We should use the class
when we only need metadata, and we should only instantiate and us an instance of `PackageBase` at build time.

Modifications:
- [x] Remove the `spack.repo.get` convenience function (which was used in many places, and not really needed)
- [x] Use `spack.repo.path.get_pkg_class` wherever possible
- [x] Try to route most of the need for `spack.repo.path.get` through `Spec.package`
- [x] Introduce a non-data descriptor, that can be used as a decorator, to have "class level properties"
- [x] Refactor unit tests that had to be modified to reduce code duplication
- [x] `Spec.package` and `Repo.get` now require a concrete spec as input
- [x] Remove `RepoPath.all_packages` and `Repo.all_packages`

lib/spack/llnl/util/lang.py

@@ -1072,3 +1072,15 @@ def __exit__(self, exc_type, exc_value, tb):
         # Suppress any exception from being re-raised:
         # https://docs.python.org/3/reference/datamodel.html#object.__exit__.
         return True
+
+
+class classproperty(object):
+    """Non-data descriptor to evaluate a class-level property. The function that performs
+    the evaluation is injected at creation time and take an instance (could be None) and
+    an owner (i.e. the class that originated the instance)
+    """
+    def __init__(self, callback):
+        self.callback = callback
+
+    def __get__(self, instance, owner):
+        return self.callback(owner)