If main script is specified as a 'package:' URI, kernel compiler
should not try to convert it to 'package:' URI using --packages file.
Bug: FL-186
Change-Id: I65e8825470ca08b5dd1ae5bd30cf4f5699d6d03c
Reviewed-on: https://dart-review.googlesource.com/c/93365
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>