Add git submodules support

[wbthomason]

No description provided.

[matu3ba]

git submodules might break your simple method of deduplication.
packer supports 1. git repos, 2. luarocks (which in turn supports arbitrary installments and compiling of projects like luaformatter).
Also I dont see how plugins cant vendor their own makefiles to fetch dependencies, if necessary.

Also YAGNI if no concrete use case(s) exist(s).

[wbthomason]

Plugins can vendor their own makefiles; submodules would cover a common use case for this and eliminate the need to manually specify e.g. run = 'make' for plugins which take this approach. It shouldn't interact with deduplication - plugins would be expected to structure their submodules such that any submodule dependencies are isolated and do not cause duplicate collisions (e.g. no adding submodules on a runtime sourced path).

re: YAGNI: Well, there's a reason I made this feature note almost a year ago and haven't prioritized implementing it 😄