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.mock/packages/git-test/package.py

@@ -0,0 +1,10 @@
+from spack import *
+
+class GitTest(Package):
+    """Mock package that uses git for fetching."""
+    homepage = "http://www.git-fetch-example.com"
+
+    version('git', git='to-be-filled-in-by-test')
+
+    def install(self, spec, prefix):
+        pass