Move whatsapp_find to its own file

The search of whatsapp_find is now the function whatsappFind in
whatsapp_find.go, with its matches, its fold of the names, and its
answer. It owns no state: it gets a WhatsAppLink for the device, the
client, and the own IDs, and the WhatsAppGroups for the names of the
groups. WhatsApp.Find stays the entry point of the tool and gives the
current link. The tests of the tool move to whatsapp_find_test.go. The
behavior does not change.

Issue: 6f69ca1