Remove the dependency on `package:matcher`

This package will be discontinued. We could consider adding an API based on `checks`, but I think it would be better to use `bool Function(T)` and we can (in some other place) have utilities to easily convert a `Matcher` or a `Condition` to the predicate callback.

I think most of the places that accept a `Matcher` today currently are typed as `Object`, so we can probably add support for the predicate, migrate usages that must roll atomically to use that API, then publish the breaking change that makes the signature a function type.