Nested packages should be ignored in the surrounding package

[dcharkes]

Dependency validator reports dependencies in nested packages. However, code in a nested package should be its own package and not contribute to the surrounding package.

In a fresh checkout of https://github.com/dart-lang/native:

$ dart pub global run dependency_validator -C pkgs/code_assets

Validating dependencies for code_assets...
These packages are used outside lib/ but are not dev_dependencies:
  * archive
  * ffi
  * ffigen
  * host_name
  * http
  * mini_audio
  * native_toolchain_c
  * pointycastle
  * sqlite
  * sqlite_prebuilt
  * stb_image

Packages such as ffigen are only used in pkgs/code_assets/example/host_name/tool/ffigen.dart and pkgs/code_assets/example/host_name/ has it's own pubspec.yaml, it is its own package.