[flutter] Use standard checkout layout

Flutter engine's documented checkout layout suggests to put it in an
'engine' subdirectory. Using this layout may resolve a class of errors
where Flutter tooling does not respect the local engine paths correctly.

Change-Id: I4f755d570e3a67621336532c846670aab164ab07
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/175080
Commit-Queue: Alexander Thomas <athom@google.com>
Reviewed-by: Karl Klose <karlklose@google.com>
7 files changed