to support complete for rosidl and avoid calling multiple compinit

[iuhilnehc-ynos]

Signed-off-by: Chen Lihui [email protected]

[iuhilnehc-ynos]

to fix ros2/ros2_documentation#2944

[mjbogusz]

Cross-ref ros2/ros2cli#534

[mjbogusz]

If I understand correctly, this solution relies on checking whether functions compdef and complete exists and assumes that if so, then they must have been called.
This MIGHT conflict with other systems/scripts that either utilize completion or define functions with such names. For example I have NVM installed (note: the nvm script merely calls bashcompinit):

$ type compdef
compdef is a shell function from /usr/share/zsh/functions/Completion/compinit
$ type complete
complete is a shell function from /home/mjbogusz/.nvm/bash_completion

Note however that this might not be a problem at all, I'm just documenting my observations.