Ignore nested packages in surrounding package (#173) - #183

AI-generated -- I will take a look at this myself first, don't read yet :)

Motivation

Fixes #173.

Dependency validator previously scanned all files outside lib/ and bin/ recursively and attributed all Dart/SCSS/Less dependencies to the surrounding package. When a package contains nested packages (e.g. example/host_name/ with its own pubspec.yaml), code in the nested package was incorrectly analyzed as part of the surrounding package, causing false-positive missing dev_dependencies warnings for packages only used within the nested package.

Changes

• Added listNestedPackages helper to discover all nested package directories (subdirectories containing pubspec.yaml, excluding the root package and hidden directories like .dart_tool/).
• Excluded nested packages when listing public and non-public files in the surrounding package.
• Updated unusedDependencies handling so analyzer warning is checked before removing ignored packages.
• Updated test dependency constraints in executable_test.dart for modern analyzer compatibility.
• Added comprehensive unit and integration tests in test/utils_test.dart and test/nested_packages_test.dart.
• Added unreleased changelog entry.

Testing/QA Instructions

Automated tests have been added covering nested package handling and listNestedPackages.
All unit and integration tests pass via dart test.