refactor: move `//labgrid/python` to `//labgrid:pkg`
BREAKING CHANGE: `//labgrid/python` is now `//labrid:pkg` (or `//labgrid`) This mirrors what a `pip.parse` hub labels have.
BREAKING CHANGE: `//labgrid/python` is now `//labrid:pkg` (or `//labgrid`) This mirrors what a `pip.parse` hub labels have.