* guix/scripts/lint.scm (origin-uris): New procedure.
(check-source): Use it.
(check-mirror-url): New procedure.
(%checkers): Add 'mirror-url' checker.
* tests/lint.scm ("mirror-url")
("mirror-url: one suggestion"): New tests.
* doc/guix.texi (Invoking guix lint): Document it.