(#460) Extend license mapping for common SPDX license names (#509)

Fixes #460 
Signed-off-by: Christopher Angelo Phillips <christopher.phillips@anchore.com>

internal/spdxlicense/license.go

@@ -4,7 +4,15 @@ import (
 	"strings"
 )
 
-//go:generate go run generate_license_list.go
+// https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/#license-short-name
+// License generated in license_list.go uses a regular expression to help resolve cases where
+// x.0.0 and x are supplied as version numbers. For SPDX compatibility, versions with trailing
+// dot-zeroes are considered to be equivalent to versions without (e.g., “2.0.0” is considered equal to “2.0” and “2”).
+// EX: gpl-2+ ---> GPL-2.0+
+// EX: gpl-2.0.0-only ---> GPL-2.0-only
+// See the debian link for more details on the spdx license differences
+
+//go:generate go run generate/generate_license_list.go
 
 func ID(id string) (string, bool) {
 	value, exists := licenseIDs[strings.ToLower(id)]