Refactor providers/managers

Refactor how providers/managers are implemented in LMS, substantially
simplifying code, and opening up further simplification around
permissions in the next change.

Also, limit the number of additional packages providers can have, and
merge WorkSource into ProviderRequest so that providers can pass just
ProviderRequests around, not both.

Test: presubmits
Change-Id: Ic6fab46883cf53eb26368b54a824feac39c57a32