Fine. Extract only LibraryManifestPrinter and BundleRequirementsPrinter.

This groups code related to printing these objects into its own
namespace, and allows further split into even smaller methods. So,
it is easier to read and evolve.

Change-Id: I5e92736689232fc9db121580b135b0959f140ff8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/417141
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov