Allow setting micropip `index_urls` to Mahoraga

[hingebase]

Currently we only support passing absolute wheel urls to micropip.install. To support index_urls, the following issues need to be solved, either by a monkey patch over micropip, or by avoiding such use cases carefully:

• My attempt to use index_urls for installing a scikit-learn wheel from pypi.anaconda.org pyodide/micropip#80 (comment)
• Transform relative index url in to absolutes one pyodide/micropip#150 (comment)