Let all imports come from toplevel module, as one line

[emersonknapp]

That way instead of

from launch_frontend_py import launch
from launch_frontend_py.actions import executable

A user only has to remember a single module import

from launch_frontend_py import launch, executable

Highlighted changes:

• Collapsed the contents of action.py in to __init__.py to simplify the handling of __all__ for that toplevel module.
• Add make_valid_name to handle the underscore suffixing, so it's done the same way both places it's used