Use consistent license file crawling

Instead of relying on trivy or askalono to crawl a directory for license files, it'd be ideal to have our own crawl function and then pass its result on to the underlying detector implementation. This way, go_vendor_license install / %go_vendor_license_check will behave consistently no matter whether askalono, trivy, or none of the above (see https://gitlab.com/gotmax23/go-vendor-tools/-/issues/3) are installed.