Setup fewer wheel repos in pip.parse

[aignas]

Currently we create one repository per python version per wheel repo in any hub that is configured with experimental_index_url.

If pipstar is enabled by default, we can instead just create a single repository.

TODO:

• Create a single repo for a wheel file when pipstar and index_url is used no matter how many python versions are used.
• Extract the wheel using download_and_extract (probably depends on the venv code to be stabilized).
• Parse the entry_points.txt in starlark

Won't do:

• Have a default interpreter that we use to extract wheels if pipstar is enabled.
• create py_console_script_binary targets and have everything backwards compatible.

This is just a laundry list of things related, so in the future I may split the ticket further. Just wanted to dump my brain a little and transfer some paper notes to the ticket tracker.