refactor(_command_offset): rename { => _comp}{_command_offset,_command,_root_command}

[akinomyoga]

Extracted from #791.

• cd98abd move _command: This just moves the position of the function. I think _command_offset had been sufficiently short before but now is a long function, so it shouldn't be put between _command and complete -F _command any more.
• 15c699f rename { => _comp}{_command_offset,_command,_root_command}: Naming convention suggested by Customizability and naming conventions for each completion #539.

The description is unchanged.