concretizer: swap priority of selecting provider and default variant (#20182)

refers #20040

Before this PR optimization rules would have selected default
providers at a higher priority than default variants. Here we
swap this priority and we consider variants that are forced by
any means (root spec or spec in depends_on clause) the same as
if they were with a default value.

This prevents the solver from avoiding expected configurations
just because they contain directives like:

depends_on('pkg+foo')

and `+foo` is not the default variant value for pkg.

var/spack/repos/builtin.mock/packages/dep-with-variants/package.py

@@ -0,0 +1,15 @@
+# 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 DepWithVariants(Package):
+    """Package that has a variant which adds a dependency forced to
+    use non default values.
+    """
+    homepage = "https://dev.null"
+
+    version('1.0')
+
+    variant('foo', default=False, description='nope')
+    variant('bar', default=False, description='nope')
+    variant('baz', default=False, description='nope')