Get how each line of a PCB version's bill of materials is sourced.

A line's sourcing default is either macrofab, where MacroFab purchases a part from the
line's approved vendor list (turnkey), or customer, where you supply a part from that list.
New lines default to macrofab.

PCB quotes and the ready check use each line's sourcing default unless the request passes
part_origins to override it. Whether a line is placed at all is controlled separately, by
the BOM line's populate flag.