* fix tau installation issue : setup_environment() is
called before install phase when 'Makefile.*' doesn't
exist (causing list index out of range error).
* Added detailed comment suggested by @alalazo
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).
