Fix: don't issue tasks for parent schema definition documents in `schema` task

[sandreae]

📋 Checklist

• [x] Add tests that cover your changes
• [x] Add this PR to the Unreleased section in CHANGELOG.md
• [x] Link this PR to any issues it closes
• [x] New files contain a SPDX license header

[codecov[bot]]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (a24b2d8) 92.46% compared to head (8e72b5e) 92.69%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #581 +/- ## ========================================== + Coverage 92.46% 92.69% +0.23% ========================================== Files 106 106 Lines 18400 18384 -16 ========================================== + Hits 17013 17041 +28 + Misses 1387 1343 -44 ``` | [Files](https://app.codecov.io/gh/p2panda/aquadoggo/pull/581?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda) | Coverage Δ | | |---|---|---| | [aquadoggo/src/materializer/tasks/dependency.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/581?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9tYXRlcmlhbGl6ZXIvdGFza3MvZGVwZW5kZW5jeS5ycw==) | `97.39% <100.00%> (-0.14%)` | :arrow_down: | | [aquadoggo/src/materializer/tasks/schema.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/581?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9tYXRlcmlhbGl6ZXIvdGFza3Mvc2NoZW1hLnJz) | `96.26% <97.18%> (+8.03%)` | :arrow_up: | | [aquadoggo/src/materializer/tasks/blob.rs](https://app.codecov.io/gh/p2panda/aquadoggo/pull/581?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda#diff-YXF1YWRvZ2dvL3NyYy9tYXRlcmlhbGl6ZXIvdGFza3MvYmxvYi5ycw==) | `84.25% <84.61%> (+28.05%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/p2panda/aquadoggo/pull/581/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=p2panda)

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