MapgenBasic node resolver: Various fixes

MapgenBasic node resolver: Various fixes

Add a fallback node for stair_desert_stone to avoid ignore placed
in Minimal subgame desert dungeons.
Don't allow river_water_source to fallback to water_source as river
water needs to be non-renewable and have a short flow range.
Make stair_sandstonebrick fall back to sandstonebrick instead of
sandstone.

Re-order some lines.
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