Fix Importer's available resources checked states

[ptsavol]

When both Tool and a Data Connection are connected to an Importer, the connection from Tool -> Importer, clears the Importer's available resources list. This is why Importer's available resource checked states we're not loaded correctly from project.json when opening a project.

Fixes spine-tools/Spine-Toolbox#2315

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
• [ ] Unit tests pass

[codecov-commenter]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.03% :warning:

Comparison is base (90c8780) 55.46% compared to head (e113ceb) 55.43%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #158 +/- ## =========================================== - Coverage 55.46% 55.43% -0.03% =========================================== Files 192 192 Lines 17286 17287 +1 Branches 2829 2830 +1 =========================================== - Hits 9587 9583 -4 - Misses 7179 7183 +4 - Partials 520 521 +1 ``` | [Files Changed](https://app.codecov.io/gh/spine-tools/spine-items/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools) | Coverage Δ | | |---|---|---| | [spine\_items/importer/importer.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvaW1wb3J0ZXIvaW1wb3J0ZXIucHk=) | `51.56% <50.00%> (-0.24%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/spine-tools/spine-items/pull/158/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools)

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[ptsavol]

Closing this PR without merging. The fix will be on Spine Toolbox side instead.