Needed to find bottlenecks in fns, so break out individual sections of
the massive function for easier understanding.
buildItemSubgraphs (one section of the previous function) would build
millions of connections that were never used as stacked pins created X!
connections. Also tested using sets instead of lists and keeping unique
lists to avoid flagging but none of these were as performant as using
flags to remember which items had already been processed.
Fixes https://gitlab.com/kicad/code/kicad/issues/10974
(cherry picked from commit 17b1b68ac7)
Seth Hillbrand
jean-pierre charras
Jeff Young