New, cleaner package repository structure.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

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

@@ -0,0 +1,24 @@
+from spack import *
+import os
+
+
+class Global(Package):
+    """ The Gnu Global tagging system """
+    # FIXME: add a proper url for your package's homepage here.
+    homepage = "http://www.gnu.org/software/global"
+    url = "http://tamacom.com/global/global-6.5.tar.gz"
+
+    version('6.5', 'dfec818b4f53d91721e247cf7b218078')
+
+    depends_on('exuberant-ctags')
+
+    def install(self, spec, prefix):
+        config_args = ['--prefix={}'.format(prefix)]
+
+        config_args.append('--with-exuberant-ctags={}'.format(
+            os.path.join(spec['exuberant-ctags'].prefix.bin, 'ctags')))
+
+        configure(*config_args)
+
+        make()
+        make("install")