parallelize pip wheel

[mmerickel]

What's the problem this feature will solve?
pip wheel -r requirements.txt may trigger building wheels for a lot of packages at once and currently every package is converted to a wheel sequentially which may take a lot of time. Especially packages that override the build step to run custom scripts.

Describe the solution you'd like
pip wheel should automatically build packages in parallel, or with a --parallel option on the CLI.

Additional context
I'm not clear in what scenarios would cause problems building packages in parallel into wheels. It seems most cases would not be an issue because packages can be built without being installed, indicating that they can be built in isolation.

[pradyunsg]

This has been brought up in #8187, but I'll keep this open as a separate feature request. There's a lot of related discussion in that issue, which is definitely worth a read if you're interested in this.

As far as I can tell, this would involve a fair amount of work, and it's not immediately clear what the implications of such a change might be. Given the (almost entirely) volunteer nature of developer time toward pip, I wouldn't recommend holding your breath in terms of how quickly this gets done. :)