* root: fix variant detection for external
A few fixes (possibly non-exhaustive) to `spack external find root`
Several variants have had `when=` clauses added that need to be
propagated to determine_variants. The previously used
Version.satifies("") method also has been removed, it seems. It's
slightly cumbersome that there is no self.spec to use in
determine_variants, but comparisons using Version(version_str) work at least
* remove debug printout
590be9bba1