submodule--helper: rename compute_submodule_clone_url()

submodule--helper: rename compute_submodule_clone_url()

Let's rename 'compute_submodule_clone_url()' to 'resolve_relative_url()'
to make it clear that this internal helper need not be used exclusively
for computing submodule clone URLs.

Since the original 'resolve-relative-url' subcommand and its C entry
point has been removed in c461095ae3 (submodule--helper: remove
resolve-relative-url subcommand, 2021-07-02), this rename can be done
without causing any confusion about which function it actually binds to.

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