submodule: always validate gitdirs inside submodule_name_to_gitdir

submodule: always validate gitdirs inside submodule_name_to_gitdir

Move the ad-hoc validation checks sprinkled across the source tree,
after calling submodule_name_to_gitdir() into the function proper,
which now always validates the gitdir before returning it.

This simplifies the API and helps to:
1. Avoid redundant validation calls after submodule_name_to_gitdir().
2. Avoid the risk of callers forgetting to validate.
3. Ensure gitdir paths provided by users via configs are always valid
   (config gitdir paths are added in a subsequent commit).

The validation function can still be called as many times as needed
outside submodule_name_to_gitdir(), for example we keep two calls
which are still required, to avoid parallel clone races by re-running
the validation in builtin/submodule-helper.c.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>