I tried adding it just like that to my config.scm declaration:
--8<---------------cut
here---------------start------------->8---
(packages
(append
(map specification->package
'("git"
"git:send-email"
...)
--8<---------------cut
here---------------end--------------->8---
But when running guix system reconfigure, it complains about it,
saying:
--8<---------------cut
here---------------start------------->8---
guix system: error: git:send-email: unknown package
guix system: error: failed to load 'guix.scm':
ice-9/boot-9.scm:222:17: In procedure map1:
--8<---------------cut
here---------------end--------------->8---