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

In https://github.com/nexB/scancode-toolkit/issues/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 https://github.com/nexB/scancode-results-analyzer/pull/46)