Support composing multibinders for private modules

When creating a multibinder with the same key as the parent module in a PrivateModule, guice produces an error. This check avoids the confusing situation of having different multibindings based on the location of injection, but it creates a problem: if a private module wants to "inherit" all of the multi-bindings from the parent in its own multibinder with a different key, it must manually re-state each binding. This is error-prone and cumbersome.

To support this use case, we could allow a multibinder to copy/inherit all the bindings from another multibinder of the same type. e.g. `LinkedBindingBuilder inheritBindings(Multibinder multibinder)`

Motivation: Multibinders are often used with AOP type patterns. I recently found a case in our code base where private datasource modules were not getting important execution listeners wired up to terminate long running database queries!