code: protected Proto searchFromProtoPathAndClasspath(String path, Proto importer)
              throws Exception
      {
          // proto_path
          File protoFile;
          for (File dir : __protoLoadDirs)
          {
              if ((protoFile = new File(dir, path)).exists())
                  return loadFrom(protoFile, importer);
          }
  
          // classpath
          Proto protoFromOtherResource = loadFromOtherResource(path, importer);
          if (protoFromOtherResource == null)
          {
              throw new IllegalStateException("Imported proto " + path +
                      " not found. (" + importer.getSourcePath() + ")");
          }
  
          return protoFromOtherResource;
      }
docs: Search from proto_path and classpath (in that order).
  <p>
  
  <pre>
  Enable via:
  -Dproto_path=$path -Dproto_search_strategy=2
  </pre>