submodule--helper: refactor module_clone()

submodule--helper: refactor module_clone()

Separate out the core logic of module_clone() from the flag
parsing---this way we can call the equivalent of the `submodule--helper
clone` subcommand directly within C, without needing to push arguments
in a strvec.

Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>