DetectCustomPostTypeURLs returns URLs for almost all post types

This is a problem because that includes built-in ones such as `Post` - making this detector return a potentially HUGE list of duplicate URLs to the `DetectPostURLs` detector.

There are a few things we could do here.
* Exclude all built-in post types OR
* Rename the class to DetectAllPostTypeURLs and just cover everything in this one class.