concretizer: try hard to obtain all needed variant_possible_value()'s (#20102)

Track all the variant values mentioned when emitting constraints, validate them
and emit a fact that allows them as possible values.

This modification ensures that open-ended variants (variants accepting any string 
or any integer) are projected to the finite set of values that are relevant for this 
concretization.

var/spack/repos/builtin.mock/packages/singlevalue-variant-dependent-type/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 SinglevalueVariantDependentType(Package):
+    """Simple package with one dependency that has a single-valued
+       variant with values=str"""
+
+    homepage = "http://www.example.com"
+    url      = "http://www.example.com/archive-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    depends_on('singlevalue-variant fum=nope')

var/spack/repos/builtin.mock/packages/singlevalue-variant/package.py

@@ -0,0 +1,19 @@
+# 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 SinglevalueVariant(Package):
+    homepage = "http://www.llnl.gov"
+    url = "http://www.llnl.gov/mpileaks-1.0.tar.gz"
+
+    version(1.0, 'foobarbaz')
+
+    variant(
+        'fum',
+        description='Single-valued variant with type in values',
+        default='bar',
+        values=str,
+        multi=False
+    )