add warning of future removal from Perl core

Module::Pluggable, along with a slew of other libraries added during the 5.9.x series to support CPANPLUS, is scheduled for removal for the core along with CPANPLUS itself.

This commit will add a warning when Module::Pluggable is used from its installed-with-perl location on perls 5.17.0 and greater.  Once the user installs Module::Pluggable from CPAN, the warning will stop.

Once this module is released and merged into core, no further updates to Module::Pluggable in core are likely to occur.  It would be great if this could be released "soon" so it can be merged back to core ASAP.

Thanks!