Consume impellerc depfiles for shader bundles

[bdero]

Summary

• probe impellerc --help before passing --depfile so older compilers that do not support the flag continue to work
• pass a bundle depfile path when supported, parse emitted dependencies, and merge them into buildOutput.dependencies
• keep the existing manifest/direct shader dependency scan as the fallback for old or no-op impellerc builds
• document the behavior and add parser/argument/probe tests

Closes #15.

Verification

• dart analyze
• flutter test
• dart format --output none --set-exit-if-changed $(find . -name '**.dart' -not -path '/build/')
• flutter pub outdated
• flutter pub publish --dry-run