Don't inherit from Exporter, but import its 'import'

This requires Exporter 5.57 which is in core since 5.8.3, but is also on
CPAN.
Note that Module::Pluggable already depends on File::Spec 3.00 that
appeared in core only in 5.8.6, so this doesn't change the dependencies
for the crazy people that avoid CPAN (do they still exist?).
Merge pull request #5 from dolmen/use-Exporter-5.57

Don't inherit from Exporter, but import its 'import'