perl-functions.eclass: Add perl_has_module

perl-functions.eclass: Add perl_has_module

This is an incredibly fast way to check if Perl considers a module
of the given name installed in any capacity, including broken.

As long as "Foo.pm" is somewhere in @INC, `perl_has_module Foo` will
return true.

Even `perl_has_module threads` will return true on non-threaded perls,
due to that module still being present, and the module only fataling
when loaded.

Whereas `perl_has_module_version threads 0` will always fail on
non-threaded perls.