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

"git send-email" learned "--reply-to=<address>" option.

* cl/send-email-reply-to:
  send-email: support separate Reply-To address
  send-email: rename variable for clarity

Code refactoring.

* np/send-email-header-parsing:
  send-email: extract email-parsing code into a subroutine

"git send-email" learned to complain when the batch-size option is
not defined when the relogin-delay option is, since these two are
mutually required.

* xz/send-email-batch-size:
  send-email: error out when relogin delay is missing

In some projects contributions from groups are only accepted from a
common group email address. But every individual may want to receive
replies to her own personal address. That's what we have 'Reply-To'
headers for in SMTP. So introduce an optional '--reply-to' command
line option.

This patch re-uses the $reply_to variable. This could break
out-of-tree patches!

Signed-off-by: Christian Ludwig <chrissicool@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The SMTP protocol has both, the 'Reply-To' and the 'In-Reply-To' header
fields. We only use the latter. To avoid confusion, rename the variable
for it.

Signed-off-by: Christian Ludwig <chrissicool@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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>

The Net::SMTP and Net::Domain were both first released with perl
v5.7.3[1], since my d48b284183 ("perl: bump the required Perl version
to 5.8 from 5.6.[21]", 2010-09-24) we've depended on 5.8, so there's
no reason to conditionally require them anymore.

This conditional loading was initially added in
87840620fd ("send-email: only 'require' instead of 'use' Net::SMTP",
2006-06-01) for Net::SMTP and 134550fe21 ("git-send-email.perl - try
to give real name of the calling host to HELO/EHLO", 2010-03-14) for
Net::Domain, both of which predate the hard dependency on 5.8.

Since they're guaranteed to be installed now let's "use" them
instead. The cost of loading them both is trivial given what
git-send-email does (~15ms on my system), and it's better to not defer
any potential loading errors until runtime.

This patch is better viewed with -w, which shows that the only change
in the last two hunks is removing the "if eval" wrapper block.

1. $ parallel 'corelist {}' ::: Net::{SMTP,Domain}
   Data for 2015-02-14
   Net::SMTP was first released with perl v5.7.3

   Data for 2015-02-14
   Net::Domain was first released with perl v5.7.3

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

The build procedure for perl/ part has been greatly simplified by
weaning ourselves off of MakeMaker.

* ab/simplify-perl-makefile:
  perl: treat PERLLIB_EXTRA as an extra path again
  perl: avoid *.pmc and fix Error.pm further
  Makefile: replace perl/Makefile.PL with simple make rules

When the batch size is neither configured nor given on the command
line, but the relogin delay is given, then the current code ignores
the relogin delay setting.

This is unsafe as there was some intention when setting the batch size.
One workaround would be to just assume a batch size of 1 as a default.
This however may be bad UX, as then the user may wonder why it is sending
slowly without apparent batching.

Error out for now instead of potentially confusing the user.
As 5453b83bdf (send-email: --batch-size to work around some SMTP
server limit, 2017-05-21) lays out, we rather want to not have this
interface anyway and would rather want to react on the server throttling
dynamically.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>