gnulib-tool.py: Simplify use of GLModuleTable accessors. * pygnulib/GLModuleSystem.py (GLModuleTable.__getitem__): Remove function. * pygnulib/GLImport.py (GLImport.gnulib_cache, GLImport.gnulib_comp) (GLImport.execute): Use function calls on the GLModuleTable to access module lists instead of using keys to emulate a dictionary.