resolvePackage method
Resolves the source for package:[pkg]/[libPath], or null if this
provider can't serve it.
Implementation
@override
Future<PackageSource?> resolvePackage(String pkg, String libPath) async {
var files = await _ensureLibFiles(pkg);
var src = files?[libPath];
if (src == null) return null;
return PackageSource('package:$pkg/$libPath', src);
}