init: move spack.repo global variable into its own module

- spack.repository module is now spack.repo

- `spack.repo` is now `spack.repo.path()` and loaded lazily

- Added `spack.repo.get()` and `spack.repo.all_package_names()` as
  convenience functions to simplify the new lazy interface.

- updated tests and code

lib/spack/docs/repositories.rst

@@ -445,7 +445,7 @@ Spack repo namespaces are actually Python namespaces tacked on under
 ``spack.pkg``.  The search semantics of ``repos.yaml`` are actually
 implemented using Python's built-in `sys.path
 <https://docs.python.org/2/library/sys.html#sys.path>`_ search.  The
-:py:mod:`spack.repository` module implements a custom `Python importer
+:py:mod:`spack.repo` module implements a custom `Python importer
 <https://docs.python.org/2/library/imp.html>`_.
 
 .. warning::