support for automatically fetching missing sub-projects

it would be useful to have some kind of "stub modules" which point to the actual module and get accordingly replaced during the build. this could happen via integration with version control systems and download+unpacking tools.

the qt supermodule is a prime example where this could be used: imagine you clone just qt5, and on the command line specify which qt modules you want to build. qbs fetches the requested modules, including their dependencies, and builds it all.

this task is complementary to QBS-62, which is about external dependencies.