py-cffi's .so was being built without the rpath being set. distutils
looks at the LDSHARED for which compiler to use to build the final .so.
Since it was not set, distutils fell back to the system provided
compiler. Setting it forces the shared library to be compiled with the
spack compiler (meaning that the rpath is set correctly).
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).
