Makes sure that dir/pubspec.yaml is resolved such that pubspec.lock and
.dart_tool/package_config.json are up-to-date and all packages are
downloaded to the cache.
Returns the dart program/snapshot to invoke for running descriptor
resolved according to the package configuration of the package at root
(defaulting to the current working directory). Using the pub-cache at
pubCacheDir (defaulting to the default pub cache).