fix: raise error for missing annotations in imported files

[Oyami-Srk]

Previously, for the following Cap'n Proto schemas (file ID omitted):

# a.capnp
using Java = import "/capnp/java.capnp";
$Java.package("org.capnproto.examples");
$Java.outerClassname("A");

struct Foo {
    bar @0 : import "b.capnp".Bar;
}

# b.capnp
struct Bar {
    field0 @0 :Text;
}

It would compile with illegal identifiers:

      public final ..Bar.Builder getBar() {
        return _getPointerField(..Bar.factory, 0, null, 0);
      }
      public final void setBar(..Bar.Reader value) {
        _setPointerField(..Bar.factory,0, value);
      }
      public final ..Bar.Builder initBar() {
        return _initPointerField(..Bar.factory,0, 0);
      }

This PR fixes this by adding errors when annotations in imported files are missing, preventing the illegal code from being constructed.

[dwrensha]

Thanks!