Unnecessary tasks created when uploaded resources are modified

[ThrawnCA]

The logic for determining whether a resource URL has changed (and therefore needs archiving) doesn't properly handle uploaded files, because the 'new' resource is the plain filename while the 'old' one has the full URL. Editing and saving an uploaded resource without making any further changes will result in an unnecessary archiver task, with a log message similar to:

DEBUG [ckanext.archiver.plugin] Resource url changed - will archive. id=3fbf pos=0 url="https://example.com/dataset/5bc484a7-8773-4301-b925-c7ba7ca5878c/resource/3fbf6fcb-fd65-4c73-b82d-21f53810c788/download/example.pdf"->"example.pdf"