code: public PackageSymbol enterPackage(Name fullname) {
          PackageSymbol p = packages.get(fullname);
          if (p == null) {
              Assert.check(!fullname.isEmpty(), "rootPackage missing!");
              p = new PackageSymbol(
                  Convert.shortName(fullname),
                  enterPackage(Convert.packagePart(fullname)));
              p.completer = thisCompleter;
              packages.put(fullname, p);
          }
          return p;
      }
docs: Make a package, given its fully qualified name.