Enforce uniqueness of the version_weight atom per node

fixes #22565

This change enforces the uniqueness of the version_weight
atom per node(Package) in the DAG. It does so by applying
FTSE and adding an extra layer of indirection with the
possible_version_weight/2 atom.

Before this change it may have happened that for the same
node two different version_weight/2 were in the answer set,
each of which referred to a different spec with the same
version, and their weights would sum up.

This lead to unexpected result like preferring to build a
new version of an external if the external version was
older.

var/spack/repos/builtin.mock/packages/old-external/package.py

@@ -0,0 +1,17 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+class OldExternal(Package):
+    """A package that has an old version declared in packages.yaml"""
+
+    homepage = "https://www.example.com"
+    url = "https://www.example.com/old-external.tar.gz"
+
+    version('1.2.0', '0123456789abcdef0123456789abcdef')
+    version('1.1.4', '0123456789abcdef0123456789abcdef')
+    version('1.1.3', '0123456789abcdef0123456789abcdef')
+    version('1.1.2', '0123456789abcdef0123456789abcdef')
+    version('1.1.1', '0123456789abcdef0123456789abcdef')
+    version('1.1.0', '0123456789abcdef0123456789abcdef')
+    version('1.0.0', '0123456789abcdef0123456789abcdef')