Specializes 'PackageResolver'.

The regular unit resolver does not need to resolve packages, and resolution of
packages can be done in a stricter way since we may assume that the package
group's members exist.  The package resolution is then only done while
resolving a package group.
Refactors 'resolver' name identification.
Fixes unicode output when run as a library.

This fixes a regression where output for the 'cflags' command would not be
unicode when 'bdemeta' is used as a library.