Fix build with Qt 5.15

In 88f8e646 addresseeMap was a KContacts::Addressee::Map and was changed to a QMap, the KContacts::Addressee::Map documentation says that the keys are unique.

And it looks like the keys in a map returned by contacts() are unique in each map, and each plugin uses keys that are urls specific to it (so the akonadi plugin uses akonadi urls and the ktp plugin uses ktp urls); thanks to dfaure for figuring that out.

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