Add mapping name as mapping source in importer

[PiispaH]

It is now possible to map the active mapping name to the source ref.

Fixes spine-tools/Spine-Toolbox#2223

Checklist before merging

• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.26%. Comparing base (c630744) to head (137ca24).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #423 +/- ## ========================================== + Coverage 78.23% 78.26% +0.03% ========================================== Files 72 72 Lines 9550 9564 +14 Branches 2039 2042 +3 ========================================== + Hits 7471 7485 +14 Misses 1741 1741 Partials 338 338 ```

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