[flutter_tools] Watch transitive #include headers for FragmentProgram hot reload

[bkonyi]

Fixes#186343

This PR enables hot reloading of shaders when their transitive #include dependencies are modified.

Changes:

• Pass --depfile to impellerc to generate dependency files.
• Track shader dependencies in DevelopmentShaderCompiler using DepfileService.
• Check if dependencies are modified during DevFS sync to trigger recompilation.
• Added extensive test coverage for edge cases (missing depfile, malformed depfile, non-file content).

[gemini-code-assist]

Code Review

This pull request introduces tracking of transitive shader dependencies in DevelopmentShaderCompiler to determine if shaders need recompilation during DevFS syncs. It parses depfiles generated by impellerc and exposes areDependenciesModified to check for modified dependencies. Feedback highlights a potential FileSystemException when calling dep.statSync() in areDependenciesModified, which could crash the sync process and should be wrapped in a try-catch block.

[fluttergithubbot]

An existing Git SHA, 4aa826b5ba779a1fd90b4406815cfaf6ff5ba06e, was detected, and no actions were taken.

To re-trigger presubmits after closing or re-opeing a PR, or pushing a HEAD commit (i.e. with --force) that already was pushed before, push a blank commit (git commit --allow-empty -m "Trigger Build") or rebase to continue.

[auto-submit]

autosubmit label was removed for flutter/flutter/187945, because The base commit of the PR is older than 7 days and can not be merged. Please merge the latest changes from the main into this branch and resubmit the PR.

[auto-submit]

auto label is removed for flutter/flutter/187945, Failed to enqueue flutter/flutter/187945 with HTTP 400: No merge queue found for branch 'main'.

The base branch was changed.

[fluttergithubbot]

An existing Git SHA, 53e8ca93e01b12045b668c8694e3e7b161ba01f2, was detected, and no actions were taken.

To re-trigger presubmits after closing or re-opeing a PR, or pushing a HEAD commit (i.e. with --force) that already was pushed before, push a blank commit (git commit --allow-empty -m "Trigger Build") or rebase to continue.

[auto-submit]

autosubmit label was removed for flutter/flutter/187945, because Pull request flutter/flutter/187945 is not in a mergeable state.