conf: fix reading lib+default.requires from rust2rpm.conf

The "default" feature is always implicitly defined, so it can always
be used in rust2rpm.conf. However, previous to this commit, it was not
loaded correctly if the crate did not explicitly define a "default"
feature.

rust2rpm/conf.py

@@ -95,10 +95,13 @@ class Rust2RpmConf:
             "lib.requires",
             "bin.requires",
         ]
+
+        # the "default" feature is always implicitly defined
+        if "default" not in features:
+            features.add("default")
+
         for feature in features:
             valid_keys.append(f"lib+{feature}.requires")
-        if "default" not in features:
-            valid_keys.append(f"lib+default.requires")
 
         # check section names
         for section in conf.keys():