concretizer: call inject_patches_variants() on the roots of the specs (#20203)

As was done in the old concretizer. Fixes an issue where conditionally
patched dependencies did not show up in spec (gdal+jasper)

var/spack/repos/builtin.mock/packages/conditionally-patch-dependency/package.py

@@ -0,0 +1,16 @@
+# Copyright 2013-2020 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 ConditionallyPatchDependency(Package):
+    """Package that conditionally requries a patched version
+    of a dependency."""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/patch-a-dependency-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+    variant('jasper', default=False)
+    depends_on('libelf@0.8.10', patches=[patch('uuid.patch')], when='+jasper')

var/spack/repos/builtin.mock/packages/conditionally-patch-dependency/uuid.patch

@@ -0,0 +1 @@
+patchadep