[flutter_tools] flutter build apk --release runs logic to discover packages with hooks 7 times

[mkustermann]

Ideally the code would be structured in a way that allows any tool to discover transitive package graph (and the subset thereof that has hooks in it) once. Then any following action can operate on that knowledge.

Though it seems a flutter build apk --release is currently triggering the logic that discovers all packages (namely PackageLayout._packagesWithHook) 7 times. This seems wasteful.

(Filing this here even though the corresponding code may be in flutter tools)

/cc @dcharkes