Missing mapping targets in the UI for contrib processors

Problem/Motivation

In #2333029: Extend mapping API to allow for defaults and multiple callbacks. a new hook called hook_feeds_processor_targets() was added that should be the new way of providing mapping targets. There was a clever solution in there to make it backwards compatible with processors that don't invoke this new hook. However, if the method getMappingTargets() is called twice during a request, the hook hook_feeds_processor_targets() won't be automatically fired the second time. This happens in Feeds since #2542416: Allow sources and targets to be defined but hidden. was committed. This causes targets to be reported as "Missing" in the UI when a processor is used which doesn't invoke hook_feeds_processor_targets(). See for an example #2581911-2: Missing mappings.

Proposed resolution

The static variable "feeds_feeds_processor_targets" is used to track if hook_feeds_processor_targets() is already invoked when hook_feeds_processor_targets_alter() is invoked. Clear this variable at the end of the process. This can be done at the time feeds_feeds_processor_targets_alter() is called, as the hook hook_feeds_processor_targets() is eventually invoked during _feeds_feeds_processor_targets_alter() which is called earlier.

Remaining tasks

Review the patch.

User interface changes

None

API changes

None

Data model changes

None

Patch will follow.