Make package fetching happen automatically about pyodide

[mdboom]

I'm starting to wonder if this isn't such a great idea. If we want to specify particular versions of packages, we'll need more of an API than Python's __import__ can provide. Having a separate step might make sense.

[bcolloran]

fwiw -- imo, from a UX perspective it would totally fine if users must specify which modules they want fetched at the time they load the language plugin (i think @hamilton suggested this somewhere, but not sure what issue number). I don't find it awkward or cumbersome at all, and it might even make a it a little less magical. I defer to you on this of course, but imo -- don't lose sleep on this one if it's too tough ;-)

[rth]

This now works with pyodide.runPythonAsync -- closing.