code: Module findModuleFromPackageName(String pkg) {
          int cp = pkg.indexOf(':');
          assert(cp != -1);
          String mod = pkg.substring(0, cp);
          return lookupModule(mod);
      }
docs: Find a module from a given package name. For example:
  The package name "base:java.lang" will fetch the module named "base".
  The package name ":java.net" will fetch the default module.