Revert "Used better name for python3 files and changed names in python code"

This reverts commit 3cb46bb356e3808e1f0c29f2dc54c58dcac1ed34.

Conflicts:

	.gitignore
	python3/google/protobuf/descriptor.py
	python3/google/protobuf/internal/descriptor_test.py
	python3/google/protobuf/internal/generator_test.py
	python3/google/protobuf/internal/message_test.py
	python3/google/protobuf/internal/reflection_test.py
	python3/google/protobuf/internal/service_reflection_test.py
	python3/google/protobuf/internal/test_util.py
	python3/google/protobuf/internal/text_format_test.py
	python3/setup.py
	src/google/protobuf/compiler/python3/python3_generator.cc
	src/google/protobuf/compiler/python3/python3_plugin_unittest.cc