Moved cpan search facility in Util

The search for a cpan executable is useful also for Exporting, so
it has been moved in a new function inside Pinto::Util, i.e.
Pinto::Util::find_cpanm_exe().

For refactoring, the code in Pinto::Role::Installer has been
changed to use the new function.
finalized initial implementation of Deployable