Loading data packages into MODULARIZE'd modules dynamically without --pre-js

Currently it's very hacky to do this, since data packages rely on global objects Module and LZ4.

I managed to hack this around, but I think this is a useful feature and should be supported natively.

Original context in https://github.com/emscripten-core/emscripten/issues/12074#issuecomment-699181317

My workaround is at: https://github.com/busytex/busytex/blob/master/busytex_pipeline.js#L23 https://github.com/busytex/busytex/blob/master/busytex_pipeline.js#L106