Update source when getting its root_package

Update source when getting its root_package

This way, callers who want source.root_package() don't have to remember
to call source.update() before that. Since source.update() is a noop if
the source has already been updated, there's not a reason I could see to
raise an error instead of just calling it.

The one remaining place that calls source.root_package() that is still
calling source.update() immediately before is in bin/read_manifest,
where the errors from update() are mapped to CliErrors.