Add new make_wheel test helper

[chrahunt]

While updating and trying to make our wheel metadata parsing stricter, I noticed
that a large number of our test wheels were noncompliant for various reasons (missing fields, inconsistent metadata).

After a closer look, it's actually pretty bad! The majority of our test wheels are
not automatically generated, many lack documentation, and the ones that are documented
may have documentation in one of several places.

To make it easier to generate compliant-by-default wheels, I've added a helper function
(tests.lib.wheel.wheel_builder) for generating wheel files and toggling various properties of them and a result class (tests.lib.wheel.WheelBuilder) that lets us choose at the call site how we want to consume it.

At a high level this will give us several benefits:

1. When tests are converted to use it, it will be clear what the difference is from
   a "regular" wheel file
2. As we increase metadata strictness, we can adjust the default code to align without
   needing a bunch of manual investigation and effort
3. Since the wheel result doesn't have to hit the filesystem unless asked, this may help
   speed up unit tests that need an actual wheel, but not necessarily a wheel file

To test the completeness of the interface I have locally converted most of our tests, so if this looks OK I can follow up with those changes.

[chrahunt]

Bump.

[pradyunsg]

The name of this helper (and PR's title) communicated something very different from what this PR is -- I thought it was adding a helper for the WheelBuilder class.

I suggest renaming the helper to generate_wheel.

[pradyunsg]

Overall, this sounds like a great idea to me. ^>^

---

I'd thought of something similar to this, to be a part of the "improve pip test suite" GSoC project ideas -- which tells me that it's probably a good idea, because two people came up with something similar independently. :)

My thought was to include aggregating all the wheels in tests/data and create_*_wheel* helpers, into a much more general generate_wheel function (something similar for source distributions too). Another thing we could explore is creating some sort of "cache" for these generated distributions, to reduce overhead from the generation logic. And potentially move this out into an independently tested helper library. :)

[sbidoul]

I thought it was adding a helper for the WheelBuilder class

@pradyunsg there is no WheelBuilder class anymore ;)

[chrahunt]

I suggest renaming the helper to generate_wheel.

How about make_wheel? Since we'll need to use it a bunch in some places it would be a little easier on the eyes.

[pradyunsg]

@pradyunsg there is no WheelBuilder class anymore ;)

Which contributed to why I was surprised by the naming here. :)

How about make_wheel?

LGTM.

[chrahunt]

Updated to make_wheel here.

[pradyunsg]

IIUC, we'd want to switch over to this more-and-more in our tests, especially the ones that depend on tests/data/.

[chrahunt]

That's the plan. I already have a fair number converted and I can issue PRs for them later today.

[pradyunsg]

Great!

BTW, @chrahunt: the merge commit for this is the 9000th commit on master. :)