use ~ prefix for dependency replacement

cmd/main.go

@@ -84,11 +84,20 @@ func runBuild(ctx context.Context, args []string) error {
 				return err
 			}
 			mod = strings.TrimSuffix(mod, "/") // easy to accidentally leave a trailing slash if pasting from a URL, but is invalid for Go modules
-			plugins = append(plugins, xcaddy.Dependency{
-				PackagePath: mod,
-				Version:     ver,
-			})
-			if repl != "" {
+			// prefix ~ for dependencies
+			mod, isDep := strings.CutPrefix(mod, "~")
+			if !isDep {
+				plugins = append(plugins, xcaddy.Dependency{
+					PackagePath: mod,
+					Version:     ver,
+				})
+			}
+			if repl == "" {
+				// dependencies should have a replacement, or else there is no need to specify it
+				if isDep {
+					return fmt.Errorf("missing replacement for dependency %s", mod)
+				}
+			} else {
 				// adjust relative replacements in current working directory since our temporary module is in a different directory
 				if strings.HasPrefix(repl, ".") {
 					repl, err = filepath.Abs(repl)