Switch to using only a single wheel repository

[mhsmith]

Previously: #723

It's confusing for users to have to check two places to see which packages are available, and checking both places will slow down the build.

We should move to anaconda.org, and make pypi-13.1 redirect to it. This has the advantage of having a 24/7 support team behind it. We could use the existing BeeWare repository, but maybe it would be better to use a separate Chaquopy repository to make it easy to see at a glance which packages are available for Android and for iOS. Either way, we'd probably have to ask for a storage increase.

[mhsmith]

anaconda.org is currently serving packages in a way that doesn't allow pip to cache them, which causes a noticeable performance regression. So we'll have to keep using the existing server for now (#1047).

[mhsmith]

For this release it's still worth moving everything to pypi-13.1, and filling pypi-7.0 with symlinks. it looks like this works fine with Apache indexes. Hard links would probably inflate the size of the backup

But first check that duplicate packages don’t cause problems, as noted above.

Once the packages are all in one place, update links in documentation and issue tracker, including BeeWare. And it's also worth adding a link in the error message from build-wheel (e.g. #1040).