L::Command: Remove unused "run" from import list

Signed-off-by: Niels Thykier <niels@thykier.net>
L::CheckScript: Add load_check method

Add a new method to handle loading of the check (i.e. require +
finding the run-sub).

Signed-off-by: Niels Thykier <niels@thykier.net>
lintian: Error out early if a check cannot be loaded

Previously Lintian would attempt to (re)load the check once per
package to be processed by said check.

Signed-off-by: Niels Thykier <niels@thykier.net>