Remove the sub- naming requirement

[jdevera]

The original sub would look for commands for the sub in all directories of the PATH. This new implementation does not, it only considers scripts within libexec.

Therefore, I don't really see the need to force users to use the sub-prefix any more. I'd like to explore the consequences or removing this.

Right now I'm also using libexec to store scripts that do not start with sub-, which means they won't be available as commands, but will be available for commands. With the removal of the prefixes, these would then turn visible, so there should be a way to hide these. Perhaps putting them in a different directory, like splitting libexec into lib and exec.