emacs: devel: Add lint/download commands.

[Alex Kost]

[PATCH 1/4] emacs: Add 'guix-devel-with-definition'.
[PATCH 2/4] emacs: Add 'guix-devel-download-package-source'.
[PATCH 3/4] lint: Export 'run-checkers'.
[PATCH 4/4] emacs: Add 'guix-devel-lint-package'

-- 
Alex