Use alternative index url when installing build dependencies

[ryanking13]

Close #43

This PR changes the index url for installing build dependency. The concept of this PR is explained in #43.

• pyodide-build will try to install the build dependencies from https://pypi.anaconda.org/pyodide-build/.

  • This repository contains patched numpy, scipy + common build dependencies mirrored from PyPI.
  • https://github.com/ryanking13/pyodide-build-deps is pushing packages to this package index (will transfer to Pyodide org) after this PR is approved.

• If downloading the package is not available, it will fallback to PyPI.

• When building recipes, recipes with cross-build-env / cross-build-files will keep install the native packages to hostsitepackages

  • hostsitepackages takes precedence to the sitepackages, so the build system will grab the packages from hostsitepackages not from site-packages
  • so no breaking changes in in-tree build.

---

Advantages

Pyodide xbuildenv no longer need to build the numpy and scipy or hold it in the zip file. Also upgrading the numpy and scipy will be decoupled from Pyodide release.

Drawbacks

After this PR, we need to manage a separate repository pyodide-build-deps. This repository contains a few recipes (numpy, scipy) and we need to update them when there are breaking changes in numpy or scipy.

We also need to manage other common build dependencies. These packages can be simply mirrored from PyPI, so no complex setup is needed. But it can be annoying.

[ryanking13]

This PR is ready to be reviewed now. I'll open a PR in pyodide/pyodide to test this.

[hoodmane]

I guess I owe you a review on this one too.

[ryanking13]

Yeah, a review would be helpful, thanks! I think it is almost ready, but I am still trying to make pyodide/pyodide#5309 pass the test :).