Make multidimensional entity naming more consistent

[soininen]

Toolbox' Database editor prepends 1-dimensional entity names with double underscore __. This PR makes import_functions consistent with that behavior.

Re spine-tools/Spine-Toolbox#2423

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

[codecov-commenter]

Codecov Report

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

Comparison is base (570bceb) 76.95% compared to head (0f27399) 76.97%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #313 +/- ## =========================================== + Coverage 76.95% 76.97% +0.01% =========================================== Files 74 74 Lines 9175 9181 +6 Branches 1948 1949 +1 =========================================== + Hits 7061 7067 +6 Misses 1780 1780 Partials 334 334 ```

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