Importing `cpu_dispatcher` modifies global state

[PokhodenkoSA]

Tests numba.dppl.tests fails. It seems that importing cpu_dispatcher modifies some global state. It changes behavior of numba.
Possible solution is to import cpu_dispatcher when dppl is imported.

Originally posted by @PokhodenkoSA in #84 (comment)

[PokhodenkoSA]

#90 removes back CPUDispatcher from cpu_dispatcher.py and return it to core/registry.py. When core.registry is imported the desired global state is achieved.

The global state modification is numba.core.registry.dispatcher_registry['cpu'] = CPUDispatcher. It is the modification which was performed when cpu_dispatcher was imported.