Clean-up to various pieces of Perl code we have.

* ab/perl-fixes:
  perl Git::LoadCPAN: emit better errors under NO_PERL_CPAN_FALLBACKS
  Makefile: add NO_PERL_CPAN_FALLBACKS knob
  perl: move the perl/Git/FromCPAN tree to perl/FromCPAN
  perl: generalize the Git::LoadCPAN facility
  perl: move CPAN loader wrappers to another namespace
  perl: update our copy of Mail::Address
  perl: update our ancient copy of Error.pm
  git-send-email: unconditionally use Net::{SMTP,Domain}
  Git.pm: hard-depend on the File::{Temp,Spec} modules
  gitweb: hard-depend on the Digest::MD5 5.8 module
  Git.pm: add the "use warnings" pragma
  Git.pm: remove redundant "use strict" from sub-package
  perl: *.pm files should not have the executable bit