Extend qualification should take place in file scope.

Prior to this change resolution of types would take place in the scope of the fully-qualified name of the type you were extending. This was incorrect behavior.

For example,

    extend google.protobuf.FieldOptions {
      optional Foo foo = 1;
    }

    enum Foo {}
Merge pull request #101 from square/jw/resolution-fix

Extend qualification should take place in file scope.