I'd suggest to check out the dojo toolkit which seems to meet most of your requirements. The one I'm not sure about is CoffeeScript.
dojo works with modules written in the Asynchronous Module Definition (AMD) format. It has a build system with packages and you can aggregate them in one or several files (called layers). Apparently it accepts git type repositories, more details on the build system here:
http://dojotoolkit.org/documentation/tutorials/1.8/build/
For the record, v1.9 beta is expected next month.
