Fixes #19.

After this change, if `bar.proto` contains the line

    import public foo.proto

Then it turns into

```
module Proto.Bar(..., module Proto.Foo) where

import Proto.Foo  -- unqualified, to allow the reexport of its definitions
```

The previous hacky approach was: `Proto.Bar` doesn't reexport `Proto.Foo`;
instead, if another file `baz.proto` imports `bar.proto`, *then* the codegen
of `Proto.Baz` imports `Proto.Foo` as well as `Proto.Bar`.  This approach
let us avoid module-level exports, but had two limitations:

1) It violated the principle of "strict dependencies", since `Proto.Baz`
   is importing what is essentially a transitive dependency.
2) Non-codegen modules that import a proto module can't benefit from
   `import public` statements.