fix(py_wheel): Avoid reliance on bash in py_wheel macro.

[UebelAndre]

While trying to modernize an older repo I tried building wheels on windows and ran into a failure:

Windows Subsystem for Linux has no installed distributions.

Use 'wsl.exe --list --online' to list available distributions
and 'wsl.exe --install <Distro>' to install.

Distributions can also be installed by visiting the Microsoft Store:
https://aka.ms/wlstore
Errorcode: Bash/Service/CreateInstance/GetDefaultDistro/WSL_E_DEFAULT_DISTRO_NOT_FOUND

This appears to be caused by the py_wheel_dist rule which gets caught by //.... This target should be considered a side-effect/optional target of py_wheel. To fix:

1. Mark the target as manual, so it's only built when explicitly requested.
2. Implement copying to the directory with a Python program instead of shell, so bash
   isn't required.

[rickeylev]

Looks good! Thanks!