The map contained in the parsed package config.
This maps package names to the base URIs for those packages. These are
already resolved relative to packageConfigUri, so if they're relative
they should be considered relative to
Uri.base. They're normalized to
ensure that all URLs end with a trailing slash.
urlFor should generally be used rather than looking up package URLs in this map, to ensure that code works with a package root as well as a package config.
null when using a packageRoot for resolution, or when no
package resolution is being used.
Map<String, Uri> get packageConfigMap;