[ Presubmit ] Only update .dart_tool/package_config.json when package contents don't match
[ Presubmit ] Only update .dart_tool/package_config.json when package
contents don't match

Should reduce the frequency of trivial merge conflicts due to differing
times of the "generated" property.

Change-Id: I1f481d48be0aa21c923c63aedb7d9888d8618bcc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/207503
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Commit-Queue: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk