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.
Documentation improvements
Merge pull request #278 from hashdist/hook_file_location

Only search hook file next to package.yaml