(K - might want to reassign to Swati as she worked on related code for 1425)
Currently, we only allow "match to contact" on Exernal ID IF it is configured as the Duplicate Matching field. This is impractical - as most sites will probably want to use our default First, Last, Email for manually entered input and for create contacts - AND use External ID For matching subsequent Contributions, Memberships and Event Participation imports.
We need to "elevate" External ID as an "always available" match to contact field (as we have for "Internal Contact ID").
NOTE: While implementing, please check how we handle cases where multiple "match to contact" fields are mapped - e.g. Internal Contact ID + External Contact ID. I think we should be AND'ing these matches as we do with First / Last / Email. Please also make sure this functionality is available for all 3 imports (contribs, members, participants) AND for all 3 contact types (i.e. import contributions FROM households, FROM organizations, etc.)