os: fix mv_by_cp for directories (#20154)

Author: ttytm

vlib/os/os.v

@@ -119,10 +119,15 @@ pub fn cp_all(src string, dst string, overwrite bool) ! {
 	}
 }
 
-// mv_by_cp first copies the source file, and if it is copied successfully, deletes the source file.
-// may be used when you are not sure that the source and target are on the same mount/partition.
+// mv_by_cp copies files or folders from `source` to `target`.
+// If copying is successful, `source` is deleted.
+// It may be used when the paths are not on the same mount/partition.
 pub fn mv_by_cp(source string, target string) ! {
-	cp(source, target)!
+	cp_all(source, target, true)!
+	if is_dir(source) {
+		rmdir_all(source)!
+		return
+	}
 	rm(source)!
 }