Support cweagans/composer-patches 2.x

Looks good if the tests pass.

AFAICT this should update ComposerPatchesValidator, unless cweagans/composer-patches:2.x truly is identical wrt API and safeguards. IOW: is composer-exit-on-patch-failure truly everything that's needed? 😮 That'd be incredibly lucky!

Re #5: @cweagans confirmed on Slack:

The flag was removed in favor of always dying if the intended project state can’t be reproduced from the lock file.

So basically, we do need to update ComposerPatchesValidator to only require composer-exit-on-patch-failure for v1.x of the plugin.

#7++

I think this looks great. I contemplated two nitpicks: asking for a deprecation error when using cweagans/composer-patches:^1(bad because disruptive for no good reason) or a @todo for removing the code for that version of the Composer plugin (but that's pointless: we should wait until that goes unsupported).

So: ship it! 🚢

Automatically closed - issue fixed for 2 weeks with no activity.