submodule-config: move submodule-config functions to submodule-config.c

submodule-config: move submodule-config functions to submodule-config.c

Migrate the functions used to initialize the submodule-config to
submodule-config.c so that the callback routine used in the
initialization process can be static and prevent it from being used
outside of initializing the submodule-config through the main API.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>