I have made a vanilla commandblock contraption, and decided after to start a new life in a village. I have collected all kinds of crops, potatoes, beetroots, etc. Then a villager came, when crops were ready, to collect them. I have found out later, that they only plant wheat seeds, not planting those, which were previously. I have attached 2 videos here, so take a look on them.. (ignore the voice in the videos)
Maybe intended.
Villagers use the first nonempty slot to plant crops; if the first slot is filled with seeds, they'll plant wheat until the slot is empty.
You can empty a villager by taking all items it gets using a hopper minecart in a block of a farmland.