perl: move CPAN loader wrappers to another namespace

Move the Git::Error and Git::Mail::Address wrappers to the
Git::LoadCPAN::Loader::* namespace, e.g. Git::LoadCPAN::Error. That
module will then either load Error from CPAN (if installed on the OS),
or use Git::FromCPAN::Error.

When I added the Error wrapper in 20d2a30f8f ("Makefile: replace
perl/Makefile.PL with simple make rules", 2017-12-10) I didn't think
about how confusing it would be to have these modules sitting in the
same tree as our normal modules. Let's put these all into
Git::{Load,From}CPAN::* to clearly distinguish them from the rest.

This also makes things a bit less confusing since there was already a
Git::Error namespace ever since 8b9150e3e3 ("Git.pm: Handle failed
commands' output", 2006-06-24).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-difftool.perl

@@ -13,7 +13,7 @@
 use 5.008;
 use strict;
 use warnings;
-use Git::Error qw(:try);
+use Git::LoadCPAN::Error qw(:try);
 use File::Basename qw(dirname);
 use File::Copy;
 use File::Find;