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.