Instead of expressing the intention in terms of PackageTargets, it now
takes helper functions that select the components to use for a package
target or a component target. These helpers are given the set of
AvailableTargets and they can do error checking and return failures.

This allows the build,configure,haddock,repl implementations to have
their own behaviour and error checking and reporting for selecting
targets.

The full details of error reporting are not covered yet.