Add flag to omit prefixes while keeping groups

[e3rd]

And this is my second wish. My big project uses many argparsers. For backward compatiblity, I do not want to impose the usrs to change all the flags to its subparser variants --model.x instead of --x. But I really need to keep the help text divided into groups.

This marker adds the functionality, help text displaying groups, while omitting the args prefixes. How it should be done so that you can accept the feature please?

[brentyi]

Thanks for the PR @e3rd!

For this feature specifically: we could also change the default behavior of OmitArgPrefixes to always keep the helptext groups. This is just cosmetic, so I'm fine with being a little bit prescriptive.

For that route the change might be much easier. We could delete the group_name_from_arg() function in _parsers.py and instead set group_name = arg.extern_prefix. There's probably some tests that would need to be updated.

What do you think?

[e3rd]

Yeah, this would seem to me much cleaner but I did not dare to propose it myself as I don't know all the side use cases you may be aware of.

[e3rd]

I've changed the default behaviour in another PR and this one can be closed :)