gnulib-tool.py: Reduce code duplication in file name transformations. * pygnulib/functions.py: New file for shared functions between modules. Add a function based on functions removed from GLImport and GLTestDir. Accepts a single file name instead of a list. * pygnulib/GLImport.py (GLImport.prepare): Use the new function. (GLImport.rewrite_new_files, GLImport.rewrite_old_files): Remove functions. * pygnulib/GLTestDir.py (GLTestDir.execute): Use the new function. (GLTestDir.rewrite_files): Remove functions. * pygnulib/main.py (main): Remove unused function import. Use the new function.