Path error for 'license_file'

[chinyeungli]

CRITICAL: Field license_file: Path C:/Users/abc/123/Projects/about/about_files/test.LICENSE not found

When the input has value for the license_file field, the tool will check for the existence of this file by joining the value with the base_dir.
(By design, this field is under FileTextField)
However, this field is usually used for copying the licenses from the provided license_text_location next to the ABOUT file.
As a result, it will always fail as the licenses in license_file field are probably under the license_text_location instead of the base_dir

[chinyeungli]

One solution is to extract out the fetch-license option to a subcommand.
Users need to run the fetch-license command first to generate/copy all the licenses first and then generate the ABOUT files afterward so that this code can check the existense of the license_file.