Register a package's own fonts under packages/<self>/

[passsy]

Fixes #141

loadAppFonts() registered fonts declared with a bare lib/... asset path (the idiomatic form for a package's own fonts) only under their bare family name. When the same package references those fonts with package: '<self>', Flutter resolves the family to packages/<self>/MyFont at lookup time — a name that was never registered, so Skia fell back to Ahem (empty boxes) even though the bytes were loaded.

This bit Wiredash, which declares Inter/Wirecons with lib/assets/fonts/... paths but sets package: 'wiredash' on its TextStyle/IconData.

Fix

// pubspec.yaml
fonts:
  - family: MyFont
    fonts:
      - asset: lib/fonts/MyFont-Regular.ttf   // bare lib/... path

// referenced in code as:
TextStyle(fontFamily: 'MyFont', package: 'own_package')  // -> packages/own_package/MyFont

Before: only MyFont was registered → packages/own_package/MyFont rendered as Ahem.
After: the test target's package name is read from pubspec.yaml and bare-path fonts are additionally registered as packages/<self>/MyFont, mirroring the existing packages/ asset branch.

This makes the two directions symmetric and matches what Flutter does internally when resolving package: on a TextStyle/IconData.

The app_font template already declares PrivateFont with a bare lib/... path; a new golden test now also resolves it via packages/app_font/PrivateFont so the regression can't sneak back in.