Use injectiveVarsOfType to catch dodgy type family instance binders (#17008)

Previously, we detected dodgy type family instances binders by
expanding type synonyms (via `exactTyCoVarsOfType`) and looking for
type variables on the RHS that weren't mentioned on the (expanded)
LHS. But this doesn't account for type families (like the example
in #17008), so we instead use `injectiveVarsOfType` to only count
LHS type variables that are in injective positions. That way, the `a`
in `type instance F (x :: T a) = a` will not count if `T` is a type
synonym _or_ a type family.

Along the way, I moved `exactTyCoVarsOfType` to `TyCoFVs` to live
alongside its sibling functions that also compute free variables.

Fixes #17008.

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