resolvePackageFile function
Resolves a File relative to a package root given a packageUri string
and a relativePath (e.g. 'pubspec.yaml').
Implementation
Future<File> resolvePackageFile(String packageUri, String relativePath) async {
final pkgDir = await resolvePackageDirectory(packageUri);
return File(p.normalize(p.join(pkgDir, relativePath)));
}