I prefer them in the same package. Feels pointless to create a package specifically for interfaces. This is especially redundant for teams that simply love their interface prefixes and suffixes (e.g. the "I", and "Impl" for Java).
If there's a needs to publish a set of interfaces as public API, it makes more sense to keep them in an entirely separate project and create project dependencies instead. But it all boils down to preference and convenience of the situation I suppose.
