This relands #158897 without any changes. This was reverted due to a postsubmit failure, which was fixed by #161464. Below is the original pull request's description.

This splits the Swift Package Manager feature in two:

1. SwiftPM feature: This builds plugins using SwiftPM (if the app supports it), and, uses the new app & plugin templates that support SwiftPM
2. SwiftPM app migration feature: this updates an existing Flutter iOS or macOS app to support Swift Package Manager. This feature requires the SwiftPM feature - if SwiftPM is off, the app migration is also off.

For now, both features are off by default. We plan to turn on the first feature in the next stable release. The app migration feature will be stay off by default until we have higher confidence in the migration.

See this mini design doc: #151567 (comment)

Here's the PR that updates the SwiftPM docs: flutter/website#11495

Part of #151567

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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