submodule-config.h: move check_submodule_url

submodule-config.h: move check_submodule_url

Move 'check_submodule_url' out of 'fsck.c' and into 'submodule-config.h' as
a public method, similar to 'check_submodule_name'. With the function now
accessible outside of 'fsck', it can be used in a later commit to extend
'test-tool submodule' to check the validity of submodule URLs as it does
with names in the 'check-name' subcommand.

Other than its location, no changes are made to 'check_submodule_url' in
this patch.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>