Add pigpen.extensions.core to list of requires for load-tsv and load-lazy

The prior changes to load-tsv & load-lazy would work locally, but when run as a pig script, the required pigpen namespaces wouldn't have been initialized properly in some cases. This explicitly initializes the extensions namespace.

Also updated the code that looks for whether a resource exists to look for .class files (result of AOT)

cc @jeremyrsellars