Require new-style hooks to also be marked as hook-wrappers

[nicoddemus]

As discussed in #403, implicitly considering hooks implemented as generators to be hook-wrappers broke some downstream projects, notably conda.

@bluetech:

If the yanking ends up being good enough, I'll work on a pluggy 1.2.0 which reworks the feature (probably using an explicit wrapper=True), otherwise I can do a quick 1.1.1 release which just reverts the feature.

For now I'll assume the yanks are good, but let me know if not.

Originally posted by @bluetech in #403 (comment)