(maint) Use provider property accessors in the Cron type

ParsedFiles is the only provider which actually creates an accessor to expose
the internal `@property_hash` of provider instances. This patch replaces
uses of `property_hash` in the Cron type with the `target` accessor method that
providers define for the `target` property.
Merge pull request #2425 from Sharpie/fix-cron-purging

Use provider property accessors in the Cron type