concretizer: handle conflicts with compiler ranges correctly

As reported, conflicts with compiler ranges were not treated
correctly. This commit adds tests to verify the expected behavior
for the new concretizer.

The new rules to enforce a correct behavior involve:
- Adding a rule to prefer the compiler selected for
  the root package, if no other preference is set
- Give a strong negative weight to compiler preferences
  expressed in packages.yaml
- Maximize on compiler AND compiler version match

var/spack/repos/builtin.mock/packages/bowtie/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 Bowtie(Package):
+    """Mock package to test conflicts on compiler ranges"""
+
+    homepage = "http://www.example.org"
+    url = "http://bowtie-1.2.2.tar.bz2"
+
+    version('1.3.0', '1c837ecd990bb022d07e7aab32b09847')
+    version('1.2.2', '1c837ecd990bb022d07e7aab32b09847')
+    version('1.2.0', '1c837ecd990bb022d07e7aab32b09847')
+
+    conflicts('%gcc@:4.5.0', when='@1.2.2')

var/spack/repos/builtin/packages/akantu/package.py

@@ -49,7 +49,7 @@ class Akantu(CMakePackage):
 
     extends('python', when='+python')
 
-    conflicts('gcc@:5.3.99')
+    conflicts('%gcc@:5.3.99')
     conflicts('@:3.0.99 external_solvers=petsc')
     conflicts('@:3.0.99 +python')