lintpkgsrc: make distinfo parsing stricter

There are no distinfo files anymore in which the filename starts with
'./', so remove that check.

Complain about invalid distinfo lines. This flags a few pkgsrc-wip
packages, but none in main pkgsrc.

Extract load_distinfo into a separate subroutine, improve local variable
names.

Document that only the first distinfo algorithm (currently BLAKE2s) is
ever checked.

(rillig)