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

lib/protocol/deviceid_test.proto

@@ -3,7 +3,7 @@ syntax = "proto3";
 
 package protocol;
 
-import "github.com/gogo/protobuf/gogoproto/gogo.proto";
+import "repos/protobuf/gogoproto/gogo.proto";
 
 option (gogoproto.goproto_getters_all) = false;
 option (gogoproto.sizer_all) = false;