Fix INSTALLDIRS for perl 5.12+

See [RT.perl.org #116479](https://rt.perl.org/rt3/Public/Bug/Display.html?id=116479) :
> Historically, dual-life modules have upgraded to the "perl"/"core"
> installdirs (as opposed to site, where most modules go). This kind of
> made sense before 5.12, when we made sense out of our default loading
> order, but really doesn't anymore right now. We should make all dual
> life modules install to site when installed from CPAN on 5.12+.