concretizer: treat conditional providers correctly (#20086)

refers #20040

This modification emits rules like:

provides_virtual("netlib-lapack","blas") :- variant_value("netlib-lapack","external-blas","False").

for packages that provide virtual dependencies conditionally instead
of a fact that doesn't account for the condition.

var/spack/repos/builtin.mock/packages/conditional-provider/package.py

@@ -0,0 +1,18 @@
+# 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 ConditionalProvider(Package):
+    """Mimic the real netlib-lapack, that may be built on top of an
+    optimized blas.
+    """
+    homepage = "https://dev.null"
+
+    version('1.0')
+
+    variant('disable-v1', default=False, description='nope')
+
+    provides('v2')
+    provides('v1', when='~disable-v1')
+
+    depends_on('v1', when='+disable-v1')

var/spack/repos/builtin.mock/packages/v1-provider/package.py

@@ -0,0 +1,13 @@
+# 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 V1Provider(Package):
+    """Mimic the real netlib-lapack, that may be built on top of an
+    optimized blas.
+    """
+    homepage = "https://dev.null"
+
+    version('1.0')
+
+    provides('v1')