wasm2js: Generalize to allow 2-level imports

[sbc100]

This removes any hard-coded references to env (the mangic module name used by llvm and emscripten) and instead imports from whatever module name is used in import declaration.

[sbc100]

We can't land this until emscripten starts passing the full imports object: https://github.com/emscripten-core/emscripten/pull/17806

[sbc100]

Replaces by #5114