Fixes #2137. In small keysy with keyrings give boss keys, boss doors of dungeons with keyrings are now unlocked. Tested with this plando:

{
    "settings": {
        "open_forest": "open",
        "starting_age": "adult",
        "spawn_positions": ["adult"],
        "starting_items": {
            "Hover Boots": 1
        },
        "shuffle_smallkeys": "remove",
        "key_rings_choice": "choice",
        "key_rings": ["Fire Temple"],
        "keyring_give_bk": true,
        "starting_hearts": 20
    },
    "entrances": {
        "Adult Spawn -> Temple of Time": {
            "region": "DMC Lower Local",
            "from": "GC Darunias Chamber"
        }
    }
}

This PR also introduces the methods World.keyring and World.keyring_give_bk to consolidate the checks for the various conditions required for a dungeon to have a key ring, or to have the boss key included in the key ring, respectively.

This also reverts #2045 which was intended to fix this issue but does not actually seem to do anything as far as I can tell. The approach taken there was to give the boss keys as starting items rather than unlocking the boss doors, with the reason given that having some boss doors but not others be unlocked would be misleading in RSL. Personally I think this PR's consistent behavior is a better approach overall as the locked boss door may lead players who are familiar with how keysy normally works to think they're stuck; RSL players would just have one more thing to keep in mind for identifying settings. Alternatively, we could indicate boss keysy by unlocking the door and giving the player the boss key item — keeping a boss key after using it is the vanilla behavior after all.