gnulib-tool.py: Refactor directory tree removals. * pygnulib/constants.py (rmtree): New function. * pygnulib/GLImport.py (GLImport.execute): Use it instead of calling 'rm -rf' directly or shutil.rmtree. * pygnulib/GLTestDir.py (GLTestDir.execute, GLMegaTestDir.execute): Likewise. * pygnulib/main.py (main): Likewise.