The populate-users role overwrites `identities` inventory variables

[paulbarfuss]

We ran into an issue where we wanted to set identities.targets to idm and found that the populate-users role is overwriting the identities dictionary when setting identities.groups and identities.users

  set_fact:
    identities:
      users: "{{ lookup('csvtojson', 'file=' + csv_doc_file_name + ' var=users') }}"
      groups: "{{ lookup('csvtojson', 'file=' + csv_doc_file_name + ' var=user_groups') }}"

From discussions are the issue, it would be desirable to have any imported users and groups (in this case from a csv file) merged with any static inventory users and groups. This would need to happen without overwriting other keys, such as targets and roles.