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>

perl/Git/LoadCPAN/Error.pm

@@ -0,0 +1,46 @@
+package Git::LoadCPAN::Error;
+use 5.008;
+use strict;
+use warnings;
+
+=head1 NAME
+
+Git::LoadCPAN::Error - Wrapper for the L<Error> module, in case it's not installed
+
+=head1 DESCRIPTION
+
+Wraps the import function for the L<Error> module.
+
+This module is only intended to be used for code shipping in the
+C<git.git> repository. Use it for anything else at your peril!
+
+=cut
+
+sub import {
+    shift;
+    my $caller = caller;
+
+    eval {
+	require Error;
+	1;
+    } or do {
+	my $error = $@ || "Zombie Error";
+
+	my $Git_Error_pm_path = $INC{"Git/LoadCPAN/Error.pm"} || die "BUG: Should have our own path from %INC!";
+
+	require File::Basename;
+	my $Git_Error_pm_root = File::Basename::dirname($Git_Error_pm_path) || die "BUG: Can't figure out lib/Git dirname from '$Git_Error_pm_path'!";
+
+	require File::Spec;
+	my $Git_pm_FromCPAN_root = File::Spec->catdir($Git_Error_pm_root, '..', 'FromCPAN');
+	die "BUG: '$Git_pm_FromCPAN_root' should be a directory!" unless -d $Git_pm_FromCPAN_root;
+
+	local @INC = ($Git_pm_FromCPAN_root, @INC);
+	require Error;
+    };
+
+    unshift @_, $caller;
+    goto &Error::import;
+}
+
+1;

perl/Git/LoadCPAN/Mail/Address.pm

@@ -0,0 +1,24 @@
+package Git::LoadCPAN::Mail::Address;
+use 5.008;
+use strict;
+use warnings;
+
+=head1 NAME
+
+Git::LoadCPAN::Mail::Address - Wrapper for the L<Mail::Address> module, in case it's not installed
+
+=head1 DESCRIPTION
+
+This module is only intended to be used for code shipping in the
+C<git.git> repository. Use it for anything else at your peril!
+
+=cut
+
+eval {
+    require Mail::Address;
+    1;
+} or do {
+    require Git::FromCPAN::Mail::Address;
+};
+
+1;