When the SCOPED_INCLUDES experiment is enabled, variables from included
Taskfiles are no longer merged globally. They remain in their original
Taskfile within the DAG.
Exception: flatten includes still merge variables globally to allow
sharing common variables across multiple Taskfiles.
Add Root() method to TaskfileGraph to get the root vertex (entrypoint
Taskfile). This will be used for lazy variable resolution.
Note: Tasks already have Location.Taskfile which can be used to find
their source Taskfile in the graph, so GetVertexByNamespace is not
needed.
