extend Version class so that 2.0 > 1.develop > 1.1 and develop > master > head > trunk > 9999 (#1983)

* extend Version class so that 2.0 > 1.develop > 1.1

* add concretization tests, with preferences and preferred version.

* add master, head, trunk as develop-like versions, develop > master > head > trunk

* update documentation on version comparison

var/spack/repos/builtin.mock/packages/develop-test2/package.py

@@ -0,0 +1,18 @@
+# Copyright 2013-2019 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)
+
+from spack import *
+
+
+class DevelopTest2(Package):
+    """Dummy package with develop version"""
+    homepage = "http://www.openblas.net"
+    url      = "http://github.com/xianyi/OpenBLAS/archive/v0.2.15.tar.gz"
+
+    version('0.2.15.develop', git='https://github.com/dummy/repo.git')
+    version('0.2.15', 'b1190f3d3471685f17cfd1ec1d252ac9')
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/preferred-test/package.py

@@ -0,0 +1,20 @@
+# Copyright 2013-2019 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)
+
+from spack import *
+
+
+class PreferredTest(Package):
+    """Dummy package with develop version and preffered version"""
+    homepage = "http://www.openblas.net"
+    url      = "http://github.com/xianyi/OpenBLAS/archive/v0.2.15.tar.gz"
+
+    version('develop', git='https://github.com/dummy/repo.git')
+    version('0.2.16', 'b1190f3d3471685f17cfd1ec1d252ac9')
+    version('0.2.15', 'b1190f3d3471685f17cfd1ec1d252ac9', preferred=True)
+    version('0.2.14', 'b1190f3d3471685f17cfd1ec1d252ac9')
+
+    def install(self, spec, prefix):
+        pass