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/mock_packages/_repo.yaml

@@ -1,2 +0,0 @@
-repo:
-  namespace: gov.llnl.spack.mock

var/spack/packages/_repo.yaml

@@ -1,2 +0,0 @@
-repo:
-  namespace: gov.llnl.spack

var/spack/repos/builtin.mock/repo.yaml

@@ -0,0 +1,2 @@
+repo:
+  namespace: builtin.mock