packageRoots property
A mapping from source directories to package roots that should override the normal package: URI resolution mechanism.
If a package root is a file, then the analyzer will behave as though that file is a ".dart_tool/package_config.json" file in the source directory. The effect is the same as specifying the file as a "--packages" parameter to the Dart VM when executing any Dart file inside the source directory.
Files in any directories that are not overridden by this mapping have their package: URI's resolved using the normal pubspec.yaml mechanism. If this field is absent, or the empty map is specified, that indicates that the normal pubspec.yaml mechanism should always be used.
Implementation
Map<String, String>? packageRoots;