Add copyright only once per package

In the current code, a copyright file from a package is added to every
slice of that package. This went unnoticed because the file was silently
overwritten from successive slices of the same package. However, the
openssl's copyright file is a symlink to the libssl3's copyright file,
and creating a symlink on the same path for the second time fails. Fix
it by adding a copyright file to only the first slice of a package.

internal/slicer/slicer.go

@@ -63,8 +63,6 @@ func Run(options *RunOptions) error {
 			extract[slice.Package] = extractPackage
 		}
 		arch := archives[slice.Package].Options().Arch
-		copyrightPath := "/usr/share/doc/" + slice.Package + "/copyright"
-		hasCopyright := false
 		for targetPath, pathInfo := range slice.Contents {
 			if targetPath == "" {
 				continue
@@ -81,9 +79,6 @@ func Run(options *RunOptions) error {
 				extractPackage[sourcePath] = append(extractPackage[sourcePath], deb.ExtractInfo{
 					Path: targetPath,
 				})
-				if sourcePath == copyrightPath && targetPath == copyrightPath {
-					hasCopyright = true
-				}
 			} else {
 				targetDir := filepath.Dir(strings.TrimRight(targetPath, "/")) + "/"
 				if targetDir == "" || targetDir == "/" {
@@ -95,6 +90,20 @@ func Run(options *RunOptions) error {
 				})
 			}
 		}
+	}
+
+	// Add copyright files to list of files to extract
+	for packageName, extractPackage := range extract {
+		copyrightPath := "/usr/share/doc/" + packageName + "/copyright"
+		hasCopyright := false
+		if extractInfos, ok := extractPackage[copyrightPath]; ok {
+			for _, info := range extractInfos {
+				if info.Path == copyrightPath {
+					hasCopyright = true
+					break
+				}
+			}
+		}
 		if !hasCopyright {
 			extractPackage[copyrightPath] = append(extractPackage[copyrightPath], deb.ExtractInfo{
 				Path:     copyrightPath,