added latest version of isl. Needed to get llvm/clang in the latest
trunk to build via spack, but there are more versions available.
what is the policy regarding those? Add what we need for now?
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).
