Add lenient dependency verification mode This mode has been added because dogfooding the feature showed it could be painful to udpate the dependency verification metadata file without such an option: often we want to diagnose why a dependency is here, but as soon as the dependency verification file is present, verification is active and immediately fails the build.
This effectively prevents from using the usual tooling to check where a dependency comes from. The workaround was to temporarily rename the file, run the build, then rename again, which was tedious.
So, instead of adding a mode where verification would be totally ignored, this commit introduces a mode where the errors are turned into warnings. This doesn't totally silence the problems, which makes them more visible to the developer.