DerivingStrategies should not require individual strategy extensions to be enabled

Motivation

Currently, DerivingStrategies requires individual extensions to be enabled in order to use each strategy. For example, if DerivingStrategies is enabled, but DeriveAnyClass is not, then deriving anyclass (Foo) will always fail.

This might not seem like much of a problem, but the issue is that enabling DeriveAnyClass changes the rules about which strategy is used by default—specifically, it causes the anyclass strategy to be prioritized over the newtype strategy, making GeneralizedNewtypeDeriving useless without explicitly specifying deriving newtype.

Generally speaking, I use newtype deriving far more often than I want to use anyclass deriving, so I’d like to keep DeriveAnyClass disabled to avoid it being chosen implicitly. However, I’d still like to be able to opt into the anyclass strategy explicitly when I want to use it using the syntax introduced by DerivingStrategies.

Proposal

A solution to this is simple enough: allow any strategy to be used explicitly when DerivingStrategies is enabled, and change the individual extensions to only alter how strategies are implicitly selected. I don’t think that really has any meaningful drawbacks. DerivingVia could remain its own extension, since it is never implicitly selected as a deriving strategy anyway, but I don’t really care about that one way or the other.

A potential alternative would be to add weaker versions of GeneralizedNewtypeDeriving and DeriveAnyClass, which could be named GeneralizedNewtypeDerivingStrategy and DeriveAnyClassStrategy. Those extensions would enable individual strategies for use with DerivingStrategies, but they would never be chosen implicitly. Personally, I don’t see much advantage in doing so, but it’s a more conservative approach that still solves the issue.