create `bg_api_fall` mod, and move files `falling_node.lua` and `loose_node.lua` to there from `bg_api`.
update dependencies in other mods to account for this change.
rename mods; add `bg_` prefix.
add hacky cleanup ABM (couldn't get LBM:s to work), that converts old
nodes into new ones according to this rename.
the cleanup doesn't do anything for items, though.