Move final wheel builder copy operation to wheel command

[sbidoul]

Removes one more non-build concern from WheelBuilder.build().

[chrahunt]

Technically this changes the behavior so that wheels will appear after all builds instead of as they are built, however:

1. I think that this will be necessary going forward anyway with the new resolver work, since we won't know until the end which wheels are relevant, and will have to build some wheels in the middle of resolving
2. the wheels that we do succeed in building will be available in the cache, so if an issue does happen the next invocation of the command should be fast since it will take files directly from the cache instead of rebuilding them

So LGTM

[pradyunsg]

LGTM, though this should be called out in the release notes.

[sbidoul]

I added a .feature news explaining the new behavior. I suppose it's not a significant change that needs to be called out as breaking.

[sbidoul]

@chrahunt I can continue with moving the should_unpack code path to the install command. Does that sound good?

FYI I tried removing the unpacking, but it's still necessary for WheelDistribution.get_pkg_resources_distribution, which in turn gets called by check_install_conflicts via _simulate_installation_of.

[chrahunt]

Yup, sounds good to me. Regarding get_pkg_resources_distribution, I noticed the same and am working on getting metadata directly from the wheel to accommodate it.