FR: [submodules] Create a submodule store that supports simple CRUD

[chooglen]

Supporting simple CRUD in submodule store, namely:

• Cloning a new, named submodule from a URL
• Listing submodules
• Looking up a submodule by name
• Deleting a submodule

is enough for users to manually manage their submodules and to do end-to-end testing of important ideas (like populating the working copy).

[ilyagr]

I'm assuming you are actively working on this. I thought I'd try encouraging people to mark the issues they're working on, but feel free to un-assign yourself at any time.

[chooglen]

Thanks for the reminder! I'll try to remember, but feel free to nudge me since I don't have the habit yet.

[joyously]

Your list for CRUD has "Looking up a submodule by name" for the U(update). Wouldn't some sort of pull operation be more consistent?