Implement fake completion support for multi-cmd binaries

This will allow build-bash-completion to understand what is the list
of commands that each binary supports, and then provide recursively
completion support for each command.

Note: by "fake", I mean we use hand-built output and hardcoded
"--help-completion" matching, as opposed to automatically doing it via
regular options. It could be possible in the future to use a
two-staged command line parser, but for now not sure if it's worth it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>