Problem

piplite supports passing a string or a list of strings of index URLs

but it does not yet provide a CLI flag for them, so it is currently not possible to write commands %pip install numpy --index-urls https://anaconda.org/scientific-python-nightly-wheels/simple, as extra indices are/will be ignored and NumPy 2.0.2 from the Pyodide distribution is installed instead.

Proposed Solution

Extend https://github.com/jupyterlite/pyodide-kernel/blob/36aedb97d38cf283df8c3b0d65d7d6b8e575f0ab/packages/pyodide-kernel/py/piplite/piplite/cli.py to have a simple --index-urls option that is passed to piplite.py, at first. It could be extended to --index-url or --extra-index-url options later if and when such a change is brought upstream in micropip.

Additional context

xref: pyodide/pyodide#4898 (comment), as I've been meaning to test out the rollout of CORS headers for the Anaconda.org PyPI-like index through piplite, but I found I was unable to do so, while micropip works.