Fix pivoted import mappings

[soininen]

This PR contains a more general fix to the problem of "headers only" import mappings.

Re spine-tools/Spine-Toolbox#2017

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass