The current module cannot be queried for existing pads as we
add them (because we never commit, until we finish the whole array).
Instead, pre-gather the names and check as we add, skipping any existing
names.
Note: this will not prevent arrays becoming "mis-ordered", but there
is not a lot we can do to prevent all possible errors.
Fixes: 1808706
* https://bugs.launchpad.net/kicad/+bug/1808706
The same principle could be used to skip existing ref-des'es on PCBs.
453a91f661