Feature Request: License Scanner can detect missing license

For CI/CD, we'd like a better ability to test for an expected license.

TBD how much of this needs to change license-scanner vs. just evaluating the result in the caller.

This could be related to #29 with respect to the allow-list (expected-list?) and return code, but it is fundamentally different use case to check for unwanted (not allowed) as opposed to checking for unexpected plus not-found.