Bring timestamps of (pubspec.yaml, pubspec.lock, dart_tool/package_config.json) in order even if files are not modified

[sigurdm]

@sigurdm actually, I think there is a bug here...

If I modify pubspec.yaml by adding a comment, the timestamp is updated. But pub get bails out and doesn't touch the other files because nothing actually changed. However to the IDE, pubspec.yaml now looks newer than the other files, so we think we need to run pub get.

I don't know if #1447 will fix this or if it also just uses the timestamps. But if it does fix it, it still requires all clients to adopt it, so I think it would be best if the timestamps were still made consistent?

Originally posted by @DanTup in #4588