Update iterator type to multimap to match source.
Update iterator type to multimap to match source.

Signed-off-by: Keith Rothman <1bc19627a439baf17510dc2d0b2d250c96d445a5@users.noreply.github.com>

verilog-to-routing/tatum,kmurray/tatum,verilog-to-routing/tatum,kmurray/tatum,kmurray/tatum,verilog-to-routing/tatum