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>