Add initial declpac CLI tool implementation

pkg/merge/merge.go

@@ -0,0 +1,9 @@
+package merge
+
+func Merge(packages map[string]bool) []string {
+	result := make([]string, 0, len(packages))
+	for name := range packages {
+		result = append(result, name)
+	}
+	return result
+}