Remove dependency on micromamba, pip and empack

[martinRenou]

Problem

jupyterlite-xeus relies on micromamba and pip for creating the WASM environment and empack for packing the result for WASM. We should stop that and rely on mambajs instead.

mambajs can do both the conda env solving and pip packages solving (actually much better and faster than real pip and micromamba).

[martinRenou]

Relying on this would remove all the limitations we've seen with using pip

[jtpio]

@martinRenou curious to know if there are any blockers that would prevent moving to mambajs?

If mambajs is used to do the environment solving in the lite addon, does that mean users will need to have Node.js installed?

[martinRenou]

does that mean users will need to have Node.js installed?

Yes that is the drawback. To be decided if installing micromamba or nodejs is more disruptive for the user. I feel like having node.js installed is not such a strong requirement.

[martinRenou]

Ok, another blocker I see is that we may need to remove the feature to build using local channels. e.g. this may make emscripten-forge devs's like harder.

Or in that case we could keep the code path of micromamba, but make it a "developer" feature, not a classic user feature.