This PR symlinks SwiftPM plugins in the same directory so that they're relative to each other and the FlutterGeneratedPluginSwiftPackage.

This allows plugins to depend on each other via relative paths.

For example,

Flutter/ephemeral/Packages/.packages/plugin_a --> symlink --> /local/path/to/plugin_a
Flutter/ephemeral/Packages/.packages/plugin_b --> symlink --> /path/to/.pub-cache/plugin_b

Then in plugin_b's Package.swift, you can do the following to add a dependency on plugin_a:

dependencies: [
  .package(name: "plugin_a", path: "../plugin_a"),
],

Addresses #166528 and incremental change toward #166489.

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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