mpp: use simple urls if possible

If no `secrets` is required to download a package we can use the
simple checksum: url mapping, instead of the checksum: {url: }
one.

tools/mpp.py

@@ -285,11 +285,17 @@ class ManifestFile:
         checksums = []
 
         for dep in deps:
-            checksum = dep["checksum"]
+            checksum, url = dep["checksum"], dep["url"]
+
+            secretes = dep.get("secrets")
+            if secretes:
+                data = {
+                    "url": url,
+                    "secrets": secretes
+                }
+            else:
+                data = url
 
-            data = {"url": dep["url"]}
-            if "secrets" in dep:
-                data["secrets"] = dep["secrets"]
             self.source_urls[checksum] = data
             checksums.append(checksum)