This has general utility, independent of ListPackages() being improved
to avoid unnecessary disk reads in typically-hidden directories.

This consolidates calls to parse the root project's code tree into a
single place, so as to avoid confusion about the standard, expected way
that dep's root project tree is to be created. It may not be fully
suitable for all status cases, but standardization is crucial for init
and ensure.

These should never have been global failures within ListPackages(), as
they are entirely package-specific information.

We only perform a single allocation for all []string, and are careful to
copy all possible error values via reflection.

While this would be possible to implement, it would require writing
entirely new entry logic into wmToReach(). That seems like a
sufficiently strong indication of breaking domain assumptions that it's
preferable just to drop the parameter, which was questionable in
the first place.