Import 'import' instead of inheriting it

This fixes errors when the first use of Try::Tiny happens during global
destruction on Perl >= 5.10.0.

The problem is that the inheritance caches don't get invalidated during
global destruction, and base.pm skips classes that the calling class
already inherits from, thus populating an initial empty inheritance
cache, which doesn't get invalidated when it adds the class to @ISA.