code: public PackageSymbol enterPackage(ModuleSymbol currModule, Name fullname) {
          Assert.checkNonNull(currModule);
          PackageSymbol p = getPackage(currModule, fullname);
          if (p == null) {
              Assert.check(!fullname.isEmpty(), () -> "rootPackage missing!; currModule: " + currModule);
              p = new PackageSymbol(
                      Convert.shortName(fullname),
                      enterPackage(currModule, Convert.packagePart(fullname)));
              p.completer = initialCompleter;
              p.modle = currModule;
              doEnterPackage(currModule, p);
          }
          return p;
      }
docs: Make a package, given its fully qualified name.