Expose the mockgen.generator type

[wdullaer]

It would be nice if the generator type in the mockgen package would be available publicly from its own package.

I am writing protoc plugin to add mocks to the generated code.
Since I know what the golang protoc plugin will produce, I can easily create a model.Package from the proto file.
Having only a single code generation step, executed by protoc greatly simplifies our build toolchain.

At the moment I have to copy the generator type and methods into my project, but it would be nice if I could just rely on the type from mockgen instead.

[codyoss]

At this time I don't think we will be exposing this. In the future we may consider a use case to use this code as a library but with current deps like calling out to go list this does not seem like a good idea to expose.