feat(bzlmod): support multi-platform wheel hubs [WIP]

[aignas]

With this change we add support for platform-specific wheel registration and
doing the selection of which wheel is used at build time. This supports:

• Different package versions for different platforms.
• Different requirements files per (os, arch) tuples.
• Uses os, arch specific dependency closures (leveraging the
  experimental_target_platforms).
• Use string_flags to configure how to build things (no transition support for now).

[aignas]

I think I've identified at least multiple changes that can be separate PRs:

• refactor groups to be created inside the hub repo. feat(pip_parse): support referencing dependencies to packages via hub #1856.
• add tests for the index downloader and add a flag to control wether the downloading is happening in parallel. I've noticed it to be much easier to debug. feat(bzlmod): add a flag to control if the parallel downloading is used #1854.
• use skylib for selects in the alias repo. refactor: use bazel-skylib for creating hub repo aliases #1855.
• Add multi platform requirement support with all of the available wheels registered.

[aignas]

This now needs to be reimplemented/rebased on #1875 and #1885.