Replace counterfeiter mock generator

[kate-osborn]

Counterfeiter generates mocks of interfaces that can be used in unit testing. However, these generated mocks import the package that the interface is defined in. This prevents us from using the mocks in unit tests that also import the interface package. This makes it hard for us to define interfaces on the client side.

A/C:

• Find an alternative generator that does not depend on the package where the target interface is defined.
• If no alternative exists, consider writing one.

Aha! Link: https://nginx.aha.io/features/NKG-54

[bjee19]

Consider blocking on issue #1564 until this is through.