Use `is_unknown` flag in place of having lists of identifers

[AyanSinhaMahapatra]

In aboutcode-org/scancode-toolkit#1675 revamping unknown license detection is discussed, and one of the suggestions made is adding an is_unknown flag with the usual license flags.

Once this is done, use that flag in analyzer in place of using lists of unknown license expressions.

(Similar to adding the is_license_intro flag, which is already added in scancode and subsequently being used here by #46)