Make rope work with dynamically loaded modules elsewhere.

E.g., on Fedora-derived Linux distributions `time` (which is not in
`sys.builtin_module_names` and it is dynamically loaded from
`lib-dynload/timemodule.so`) is not recognized as part of the standard
library, which makes ImportUtilsTest.test_sorting_imports_and_standard_modules2 fail (because of superfluous \n included between import of `sys` and `time` modules).

Fixes #38