Disable SwiftPM for xcode-analyze

[stuartmorgan-g]

Until flutter/flutter#172427 is resolved, xcode-analyze doesn't work as desired with SwiftPM enabled (it analyzes only the test code, not the plugin code). To avoid losing analysis coverage in the meantime, this disabled SwiftPM temporarily while running analysis.

This PR also updates build-examples to use the newer pubspec-based config option to set the SwiftPM flag state instead of setting global state, to avoid future issues where we are unintentionally bleeding flag changes across different tests, and to make local runs not impact developer machine state.

To unit test this functionality, this adds a new feature to the existing process mock system that allows running an arbitrary test callback at the ponit where a process is being run, which in this case allows reading the temporarily-modified pubspec contents at the right point in the command execution.

Fixes flutter/flutter#171442

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³