manifest: add __contains__ method

Since `__iter__` is return an iterator over the `Pipeline` objects,
the `"name" in manifest` check would not work for name or ids. Thus
provide an implemention of `__contains__` that does exactly that.

osbuild/pipeline.py

@@ -348,6 +348,9 @@ class Manifest:
                 return pl
         return None
 
+    def __contains__(self, name_or_id: str) -> bool:
+        return self.get(name_or_id) is not None
+
     def __getitem__(self, name_or_id: str) -> Pipeline:
         pl = self.get(name_or_id)
         if pl: