[feature] Extend to support external repo

[nqminhuit]

Not many projects use git submodules, however there are many microservices (or distributed system) projects use multiple git repositories. Extend gis to support this use case.

[nqminhuit]

.gitmodules:

[submodule "module-a"]
        path = module-a
        url = git@abc:username/module-a.git
[submodule "module-b"]
        path = module-b
        url = git@abc:username/module-b.git

add new file named .gis-modules under root module content should be:

path = <directory_name_from_root_module>
path = <directory_name_from_root_module>
...

• Only consider module specified in file .gis-modules otherwise, ignore
• .gis-modules should have higher precedence than .gitmodules, means when .gis-modules exists under root directory, .gitmodule will be ignored.