license issues for `gen` and `attrib`

[chinyeungli]

This line will fail https://github.com/nexB/aboutcode-toolkit/blob/develop/src/attributecode/attrib.py#L112 with the following scenario:
Input
license_expression: apache-2.0 AND custom and with no license_name provided
The license_name_list and lic_list is supposed to be a 1 to 1 matching relationship
However, if no license_name is provided in the input and only the above license_expression is provided
Since the custom is not a valid scancode license key, the fetch-license in gen will produce something like this following:

licenses:
  - key: apache-2.0
    name: Apache License 2.0
    file: apache-2.0.LICENSE
  - key: custom

As a result, it'll have 2 license_keys with 1 license_name which make the code fail. (Although the tool will prompt error to notify custom is not a valid scancode license key in the gen process)

[chinyeungli]

There is another bug. If we have the following input:

about_resource	name	license_expression	license_file
test.c	test.c	mit AND custom	custom.txt
test.h	test.h	public-domain
test.m	test.m	custom AND public-domain	custom.txt

The gen doesn't work very well as the current --fetch-license will only work if the license_file field is not present. However, this field is needed if the user have a custom license.

The current solution for this kind of case is to add the custom license key in DJE and do the --fetch-license, but we still need to think about the solution if user doesn't want to add the custom license in DJE for whatever reasons.

[chinyeungli]

This has been fixed.