core: differentiate package-level fetch URLs by args to version()

- packagers can specify two top-level fetch URLs if one is `url`
  - e.g., `url` and `git` or `url` and `svn`

- allow only one VCS fetcher so we can differentiate between URL and VCS.

- also clean up fetcher logic and class structure

var/spack/repos/builtin.mock/packages/git-svn-top-level/package.py

@@ -0,0 +1,39 @@
+##############################################################################
+# Copyright (c) 2013-2018, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/spack/spack
+# Please also see the NOTICE and LICENSE files for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class GitSvnTopLevel(Package):
+    """Mock package that uses git for fetching."""
+    homepage = "http://www.git-fetch-example.com"
+
+    # can't have two VCS fetchers.
+    git = 'https://example.com/some/git/repo'
+    svn = 'https://example.com/some/svn/repo'
+
+    version('2.0')
+
+    def install(self, spec, prefix):
+        pass

var/spack/repos/builtin.mock/packages/git-url-svn-top-level/package.py

@@ -25,12 +25,14 @@
 from spack import *
 
 
-class GitAndUrlTopLevel(Package):
+class GitUrlSvnTopLevel(Package):
     """Mock package that uses git for fetching."""
     homepage = "http://www.git-fetch-example.com"
 
-    git = 'https://example.com/some/git/repo'
+    # can't have two VCS fetchers.
     url = 'https://example.com/some/tarball-1.0.tar.gz'
+    git = 'https://example.com/some/git/repo'
+    svn = 'https://example.com/some/svn/repo'
 
     version('2.0')
 

var/spack/repos/builtin.mock/packages/git-url-top-level/package.py

@@ -0,0 +1,53 @@
+##############################################################################
+# Copyright (c) 2013-2018, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/spack/spack
+# Please also see the NOTICE and LICENSE files for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class GitUrlTopLevel(Package):
+    """Mock package that top-level git and url attributes.
+
+    This demonstrates how Spack infers fetch mechanisms from parameters
+    to the ``version`` directive.
+
+    """
+    homepage = "http://www.git-fetch-example.com"
+
+    git = 'https://example.com/some/git/repo'
+    url = 'https://example.com/some/tarball-1.0.tar.gz'
+
+    version('develop', branch='develop')
+    version('3.4', commit='abc34')
+    version('3.3', branch='releases/v3.3', commit='abc33')
+    version('3.2', branch='releases/v3.2')
+    version('3.1', tag='v3.1', commit='abc31')
+    version('3.0', tag='v3.0')
+
+    version('2.3', 'abc23', url='https://www.example.com/foo2.3.tar.gz')
+    version('2.2', sha256='abc22', url='https://www.example.com/foo2.2.tar.gz')
+    version('2.1', sha256='abc21')
+    version('2.0', 'abc20')
+
+    def install(self, spec, prefix):
+        pass