Only search hook file next to package.yaml

hook_import_dirs is meant as the sys.path inside hooks, not
the path to find hooks. So packages with a hook file shoud be
* $pkgs/package.yaml + $pkgs/package.py, or
* $pkgs/package/package.yaml + $pkgs/package/package.py, or
* $pkgs/package/package-foo.yaml + $pkgs/package/package.py.

This should fix https://github.com/hashdist/hashdist/issues/277
*sigh*.  I'd go with real_pkgname or actual_pkgname but I agree that use_name fits here.  This is minor nit-picking, thanks for the patch, merge when you're ready.