refactor(select): rename source files to follow convention

All `[ng-model]` directive source files under `lib/directive` have a
name of the form `ng_model*.dart`. Rename:
- `input_select.dart` to `ng_model_select.dart`.
- `input_select_spec.dart` to `ng_model_select_spec.dart`.

Closes #871
Expression extractor transformer URI improvements

- Adding support for resolving 'packages/' uris (previously only handled '/packages/') from annotation templateUris.
- Adding support for /packages/ uris to the html_files transformer parameter for referencing package-relative URIs.
- Generating warnings for not-found files rather than build errors.

Closes #879
fix: expose missing NgShadowRootAware