Make %pyproject_buildrequires and %pyproject_save_files argparser errors nicer to macro users
    
    Before:
    
        %pyproject_buildrequires bogus_arg
        usage: pyproject_buildrequires.py [-h] [-r] [-w] [--wheeldir PATH] [-R]
                                          [-e TOXENVS] [-t] [-x EXTRAS]
                                          [--generate-extras] [-p PYTHON3_PKGVERSION]
                                          [-N]
                                          [requirement_files ...]
        pyproject_buildrequires.py: error: argument requirement_files: can't open 'bogus_arg': ...
    
        %pyproject_save_files
        ...
        usage: pyproject_save_files.py [-h] --output-files OUTPUT_FILES
                                       --output-modules OUTPUT_MODULES --buildroot
                                       BUILDROOT --sitelib SITELIB --sitearch SITEARCH
                                       --python-version PYTHON_VERSION
                                       --pyproject-record PYPROJECT_RECORD --prefix
                                       PREFIX
                                       varargs [varargs ...]
        pyproject_save_files.py: error: the following arguments are required: varargs
    
    After:
    
        %pyproject_buildrequires bogus_arg
        usage: %pyproject_buildrequires [-w] [-R] [-e TOXENVS] [-t] [-x EXTRAS] [-N]
                                        [REQUIREMENTS.TXT ...]
        %pyproject_buildrequires: error: argument REQUIREMENTS.TXT: can't open 'bogus_arg': ...
    
        %pyproject_save_files
        ...
        usage: %pyproject_save_files MODULE_GLOB [MODULE_GLOB ...] [+auto]
        %pyproject_save_files: error: the following arguments are required: MODULE_GLOB