Build and test namespace_package wheel for emscripten-wasm32

[hoodmane]

I will create separate PRs splitting off the changes to the actual package code.

It seems that the prebuilt emscripten-wasm toolchain doesn't come with relocations. I need to use -Zbuild-std to prevent a bunch of errors like

relocation R_WASM_MEMORY_ADDR_LEB cannot be used against symbol `__THREW__`; recompile with -fPIC

What I do not understand is why I am not needing -Zbuild-std to get cryptography to build in
pyodide/pyodide#2683
It's quite mysterious.

[hoodmane]

If we get Pyodide out of tree builds working, we could simplify this a lot and build and test all four test packages using pytest inside of Pyodide.

[davidhewitt]

It seems that the prebuilt emscripten-wasm toolchain doesn't come with relocations.

Searching for that error some more, I came across rust-lang/rust#60231 (comment). I wonder if that works?

[hoodmane]

I'm going to redo this using Pyodide instead of compiling a separate interpreter. I think this will work better.