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
Its an attribute of `PackageYAML` and used by the `PackageLoader`. to combine the hooks from all parents.