Currently, `#!python` based scripts in a wheel are put into the venv
as-is, so aren't actually runnable. Additionally, wheel entry points
aren't installed into the venv.

To fix, rewrite the scripts to re-exec themselves with the `python3`
binary in the same directory.

Also adds support for wheel entry points in venvs. This is done by
having the repo-phase parse entry_points.txt and defining a target for
each entry point. These targets run during build phase, so are able
to generate platform-specific outputs.

Cross-building with Windows is also added, supporting both Windows
as a target or exec platform to generate outputs for the target
platform.

Fixes #3202