submodule: extract path to submodule gitdir func

submodule: extract path to submodule gitdir func

We currently store each submodule gitdir in ".git/modules/<name>", but
this has problems with some submodule naming schemes, as described in a
comment in submodule_name_to_gitdir() in this patch.

Extract the determination of the location of a submodule's gitdir into
its own function submodule_name_to_gitdir(). For now, the problem
remains unsolved, but this puts us in a better position for finding a
solution.

This was motivated, at $DAYJOB, by a part of Android's repo hierarchy
[1]. In particular, there is a repo "build", and several repos of the
form "build/<name>".

This is based on earlier work by Brandon Williams [2].

[1] https://android.googlesource.com/platform/
[2] https://lore.kernel.org/git/20180808223323.79989-2-bmwill@google.com/

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>