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
I would have picked `package_name`, but I agree that the concept is confusing.  `use_name` makes me think of the virtual package name, even though I see how it's intended.

I might just add, a "for example", clause to the parameter description to ensure it is clear.