perl-module.eclass: Introduce new DIST_ control variables for EAPI=6 and avoid MY_...

perl-module.eclass: Introduce new DIST_ control variables for EAPI=6 and avoid MY_ magic

This eclass is full of magic of the type of "Set MY_xxx before inheriting,
and I'll automatically use it somehow instead of xxx". I think this is majorly
silly since 1) MY_xxx is no proper API, and 2) it was never documented anyway.
So, in EAPI=6 ignore MY_PN, MY_P, and MY_S and use DIST_PN and DIST_P
instead. In addition also replace MODULE_ control variables with their
DIST_ analogue.

The DIST_ prefix is more consistent with upstream terminology since
what we're downloading from CPAN are not actually "modules" (like CGI.pm)
but "module distributions" (like CGI.tar.gz containg a set of modules).
In addition it avoids clashes in case of kernel module build systems. We
can't use PERL_ since this is picked up by perl itself.