Only use stable versions for public mirror (#15100)

* add --skip-unstable-versions option to 'spack mirror create' which skips sources/resource for packages if their version is not stable (i.e. if they are the head of a git branch rather than a fixed commit)

* '--skip-unstable-versions' should skip all VCS sources/resources, not just those which are not cachable

var/spack/repos/builtin.mock/packages/trivial-pkg-with-valid-hash/package.py

@@ -0,0 +1,17 @@
+# 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)
+
+from spack import *
+
+
+class TrivialPkgWithValidHash(Package):
+    url = "http://www.unit-test-should-replace-this-url/trivial_install-1.0"
+
+    version('1.0', '6ae8a75555209fd6c44157c0aed8016e763ff435a19cf186f76863140143ff72', expand=False)
+
+    hashed_content = "test content"
+
+    def install(self, spec, prefix):
+        pass