The URI for the package config.
This is the URI from which packageConfigMap was parsed, if that's
available. Otherwise, it's a
data: URI containing a serialized
representation of packageConfigMap. This
data: URI should be accepted
by all Dart tools.
Note that if this is a
data: URI, it may not be safe to pass as a
parameter to a Dart process due to length limits.
null when using a packageRoot for resolution, or when no
package resolution is being used.
Future<Uri> get packageConfigUri;