Rename genpy_stubgen to genpyi

[bonprosoft]

As genmsg supposes the name for gen{name} as the language name (e.g. genpy -> py), the package name genpy_stubgen may not be a good name for the plugin. (py_stubgen is the language name for this package)
While the package user doesn't care this part, i.e. only users have to do is just adding gepy_stubgen to find_package, I feel it is not a good idea to move away from the common approach in ROS.

So I renamed the ROS package to genpyi, and will rename the repository name and python package name as well.

Other names:

• genpystub: It might be confusing with genpy-stub (the stub package for genpy)