Incorrect appstream URLs generated

For example Yakuake has org.kde.yakuake.desktop as appstream id, so the correct URL would be appstream://org.kde.yakuake.desktop

However, apps.kde.org provides appstream://org.kde.yakuake, without the .desktop suffix.

The same is the case for Dolphin, Konsole, and other apps.

Discover can handle those since plasma/discover!313 (merged), but it's still technically incorrect.

See also https://bugs.kde.org/show_bug.cgi?id=458255