packages with Hidden=true metadata are invalid

This originates from the request of downstreams to hide packages that are still installed.
why should be easier than not installing them i don't know, *but* semantically I think it makes sense act as if the package was not existing when the metadata says it's hidden

REVIEW:119329

src/plasma/package.cpp

@@ -78,6 +78,12 @@ bool Package::isValid() const
         return false;
     }
 
+    //Minimal packages with no metadata *are* supposed to be possible
+    //so if !metadata().isValid() go ahead
+    if (metadata().isValid() && metadata().isHidden()) {
+        return false;
+    }
+
     if (d->checkedValid) {
         return d->valid;
     }