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 (closed)), 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 (closed).