lint: Define some procedures for analysing code in phases.
* guix/lint.scm
(check-optional-tests): Extract logic for extracting the phases from a
package to ...
(find-phase-deltas): ... here, and ...
(report-bogus-phase-deltas): ... here.
(check-optional-tests)[check-check-procedure]: Extract code for extracting
the procedure body to ...
(find-procedure-body) ... here.
(find-phase-procedure): New procedure.
(report-bogus-phase-procedure): New procedure.
Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>