Fix handling of Debian files with trailing slash (#26087) (#26098)

Backport #26087 by @KN4CK3R

Fixes #26022

- Fix handling of files with trailing slash
- Fix handling of duplicate package file errors
- Added test for both

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

modules/packages/debian/metadata.go

@@ -80,7 +80,9 @@ func ParsePackage(r io.Reader) (*Package, error) {
 
 		if strings.HasPrefix(hd.Name, controlTar) {
 			var inner io.Reader
-			switch hd.Name[len(controlTar):] {
+			// https://man7.org/linux/man-pages/man5/deb-split.5.html#FORMAT
+			// The file names might contain a trailing slash (since dpkg 1.15.6).
+			switch strings.TrimSuffix(hd.Name[len(controlTar):], "/") {
 			case "":
 				inner = arr
 			case ".gz":