[beta] Only exclude platform directories that actually exist in analysis_options.yaml - #191624

…ions.yaml (flutter#191151)

`AnalysisOptionsMigration` wrote a fixed seven-entry `exclude` list
(`android/**`, `ios/**`, `web/**`, `windows/**`, `macos/**`, `linux/**`,
plus `build/**`) into `analysis_options.yaml` on every `flutter pub get`
/ `flutter analyze` / `flutter run` / `flutter build`, regardless of
what the project actually contains.

Two concrete problems from this:
- A Dart-only package (e.g. `dart create -t web`) that uses `web/` as a
real source directory gets it silently excluded from analysis the first
time any `flutter` command touches the package (this happens in mixed
Flutter/Dart monorepos and Dart workspaces).
- A Flutter app created with `flutter create --platforms=android,ios`
still gets exclusions for `web/`, `windows/`, `macos/`, and `linux/`,
which it doesn't have, and `ProjectMigrator` has no opt-out — every
command restores the full list.

Fix:
- `AnalysisOptionsMigration.migrate()` now skips entirely when the
package has no `flutter` dependency (i.e. it isn't a Flutter project at
all — the discriminator suggested in the issue).
- The exclude list is now built conditionally from
`project.<platform>.existsSync()` for each platform, so only directories
the project actually has as platform scaffolds are excluded. `build/**`
stays unconditional.
- `templates/app/analysis_options.yaml.tmpl` (used by `flutter create`)
gets the same per-platform conditionals via the existing mustache
context (`{{#android}}`, `{{#ios}}`, etc.), so newly created projects
don't get the fixed list either.

Fixes flutter#191131

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we expect every widget to implement].
- [ ] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] 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].
- [ ] 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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