CPAN: Fix stripping of perllocal.pod if cpan_perl_lib_path is set

For example, if cpan_perl_lib_path is set to "/usr/share/perl5", all
packages created with fpm will contain this file:

    /usr/share/perl5/x86_64-linux-gnu-thread-multi/perllocal.pod

The current code only works if cpan_perl_lib_path is undefined.

Also see #443 and #510.
Merge pull request #763 from mlafeldt/strip_perllocal_pod

CPAN: Fix stripping of perllocal.pod if cpan_perl_lib_path is set