Copy source into pyodide environment

[koenvo]

When running the tests of pyodide-http I occasionally forget to first build the wheel. It would be very convenient when the current context is copied pyodide environment before starting the test.

This could be an additional argument to run_in_pyodide like source_packages. Or maybe even more generic: copy directories to the pyodide environment before execution.

Curious what you think about it.