Generate doesn't import packages referenced in type args

[Ralph7C2]

If an interface method or function uses a generic type with a specified type parameter, like:

type SomeInterface interface {
    Do(o Optional[pkg.SomeType])
}

The generated file will use Optional[pkg.SomeType], but not import pkg.

While there is a lot more work to fully support generic interfaces, this would improve interfaces that handle other generic types.

I've created a PR that fixes this: #258