Tree pruning

[uranusjr]

This produces a ResolutionImpossible against pip install prefect[all_extras] --only-binary :all: in ~30s.

Still much to work on, mainly error reporting seems wrong. I can’t make sense of the report from the perfect example. It contains many prefect[all-extras] entries (different versions), but does not actually show where the conflict is. But the approach itself seems to work.

[uranusjr]

Also need to write comments to describe how things work.

[pradyunsg]

Approach looks great to me.

I'm a little concerned about the flexibility we're giving the provider here, but, I guess that's a ship that sailed a while back. 🙃

[uranusjr]

Forgot to mention: Please feel free to suggest alternative method names. I’m terrible at this.

[uranusjr]

This should also mention requirements_a is the new state, and requirements_b is from a known-to-fail state. This is important since the provider can implement range merging to return True when requirements_a is a subset of requirements_b to prune additional subtrees.

(The method name should probably change.)

[uranusjr]

OTOH if requirements_a is a superset of requirements_b, this should be used to exclude requirements_b from requirements_a so the resolver can avoid visiting some subtrees. Maybe the interface should be defined to perform specifier merging instead.