all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/oschwald/geoip2-golang/.gitmodules

@@ -0,0 +1,3 @@
+[submodule "test-data"]
+	path = test-data
+	url = git://github.com/maxmind/MaxMind-DB.git