A package path now consists of a namespace and a qualifier. The namespace is
either DefaultNamespace or Independent _i_, for some _i_; this is used for
independent top-level goals. Then the qualifier is either Unqualified
(default), Setup _pn_ for the setup dependencies of package _pn_, or Base _pn_,
for a dependency on base by package _pn_ (used only when we detect the presence
of a base shim). Qualifiers are not tested anymore.

This avoids non-termination in the solver. The unit tests now pass.