Refactor read_products and its callers

- Promote to public function since it's used in other plugins anyway
- Move resolving of addons to read_products (print errors immediately)
- Cache product list based on mtime; mostly to reduce the amount of spam
  from errors as performance was never an issue.
- Cache product object in cart entry, so statiegeld_tokens plugin
  doesn't have to do the lookup all over again.

lib/RevBank/Plugins.pm

@@ -53,7 +53,7 @@ sub load($class) {
         RevBank::Eval::clean_eval(qq[
             use strict;
             use warnings;
-            use feature qw(signatures);
+            use feature qw(signatures state);
             no warnings 'experimental::signatures';
             package $package;
             BEGIN { RevBank::Global->import; }