In the corner case that a package has no components then if we allow it
to be handled as normal per-component nodes then the whole package would
vanish from the plan, which breaks later error reporting. Instead we
keep 0-component packages as whole-package nodes (as we do in a few
other cases like old Cabal versions or custom Setup scripts).

Later on we need to be able to look up package names that users have
requested and find what components they contain. It's ok to report that
a package has no components, but it's not ok to report that the package
does not exist at all. Hence it's important to preserve these packages
in the plan.

If we ever move fully to a per-component plan then we will need to
revisit this issue of how to represent packages with no components.