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

[vadimkantorov]

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 #12074 (comment)

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

[vadimkantorov]

It would also be nice if there was a unified way of creating/loading traditional packages that embed the data and "virtual" packages that redirect file access to URLs. Currently this is possible, but it is not easy to switch from one to the other. It could be a mode for file_packager.py or a tool with a similar interface.