fix unknown type when using relative namespace in proto

[DualHappiness]

修复引用外部type不使用全名时会导致unknown type的问题

// a.proto
syntax = "proto3";

package com.x.a;

message A {
  int32 n = 1;
}

// b.proto
syntax = "proto3";

package com.x.b;

message B {
 a.A a = 1;
}

[starwing]

Thanks!