[tool] Deduplicate fixture builders in build_swift_package_test.dart - #190624

Replaces 17 identical occurrences of boilerplate `BuildSwiftPackageUtils` dependency injection constructor blocks with a single file-local factory helper.

* Adds top-level private `_createTestUtils` helper using named required parameters to match suite idioms.
* Removes over 140 lines of duplicate test setup code while maintaining 100% test behavior parity.
* Formats invocations to adhere to the 80-column line length lint and specifies explicit types for local variables.

No issue (purely internal test suite duplication cleanup; no functional or behavioral production changes).

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is [test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
- [x] All existing and new tests are passing.

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[Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[AI contribution guidelines]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#ai-contribution-guidelines
[Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
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[breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

…in tests

…ckage tests