This should at least partially resolve #16.
The way this is supposed to work is:

• introduce a new unknown list used in the case of a repo that doesn't have a license file
• scanning a repo that doesn't have a license still generates the error message, but overall report is OK
• in case the target repository changed the license, it will no longer match the filter introduced with the logic, meaning it will be reported as an error and the tool will exit with non-ok code, notifying us things changed

Previously these repositories would go under ignore, but then they will always be ignored and one would never know if the license changed.

Example configuration tested with opentf repository:

approved = [
  "apache-2.0",
  "bsd-2-clause",
  "bsd-3-clause",
  "isc",
  "mpl-2.0",
  "mit",
]

unknown = [
  "github.com/hashicorp/consul/api",
  "google.golang.org/protobuf"
]