path: refactor `repo_submodule_path()` family of functions

path: refactor `repo_submodule_path()` family of functions

As explained in an earlier commit, we're refactoring path-related
functions to provide a consistent interface for computing paths into the
commondir, gitdir and worktree. Refactor the "submodule" family of
functions accordingly.

Note that in contrast to the other `repo_*_path()` families, we have to
pass in the repository as a non-constant pointer. This is because we end
up calling `repo_read_gitmodules()` deep down in the callstack, which
may end up modifying the repository.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>