Support package specifiers

[WebDrake]

The proto2 specification allows for imported data structures to be specified precisely using package specifiers, as described here:
https://developers.google.com/protocol-buffers/docs/proto#packages

However, dproto fails to handle package specifiers, emitting an undefined identifier error. This makes it very difficult to have shared .proto definition files between, say, a D app and a Java app (since protoc will fail to compile for Java if the package specifiers are missing).

As an example, consider the two following .proto files:

inner.proto

package innerproto;

message InnerMessage {
    optional uint64 id = 1;
    optional string message = 2;
}

outer.proto

import "inner.proto";

message OuterMessage {
    optional uint64 id = 1;
    optional innerproto.InnerMessage message1 = 2;
    optional innerproto.InnerMessage message2 = 3;
}

If the innerproto. package specifier is removed from outer.proto, then protoc will fail to compile it for Java. OTOH dproto will fail to handle outer.proto if the package specifier is present.

[timotheecour]

@msoucy This feature would be great to correctly implement spec and interoperate with systems that use these; how much work would that be? Any pointers (eg what to change) would help!