Improve handling of license exceptions

There are cases where a notice may not be amenable to a clean detection such as when we would have:
- a gpl-2.0
- a classpath-exception-2.0

both detected separately. Yet these we could instead report a single `gpl-2.0 WITH classpath-exception-2.0`.

To achieve this we should IMHO:
1. use the upcoming LicenseDetection post-processing approach
2. tag all exceptions with the list of license keys that they would typically except
3. when we have a detected solo exception preceded by license tag or notice of its excepted license then we could return the combined expression as a new detection from the two separate detections.