[Build Tools] Help users compose their wanted components

[imkiva]

Why

Since Mozart++ had separated its modules into separate repos,
let's be good people to do it to the end.

Supposed usage

If the user wants to use core and system:

$ cd /path/to/the/project
$ mpp use core system

and the mpp program will automatically clone core and system and their dependencies
to the project third-party/ directory each as a git submodule.

So the resulting directory structure will be like:

project/
    third-party/
        mpp_core/
            ...
        mpp_system/
            ...
        ...