[C++, Go, Bazel]: Fix up Bazel support (#4672)

* Add Bazel support for Go library

* Make flatc compile again.

Was missing java_generator, which is now referenced from source.

BUILD

@@ -81,6 +81,8 @@ cc_binary(
         "grpc/src/compiler/cpp_generator.h",
         "grpc/src/compiler/go_generator.cc",
         "grpc/src/compiler/go_generator.h",
+        "grpc/src/compiler/java_generator.cc",
+        "grpc/src/compiler/java_generator.h",
         "grpc/src/compiler/schema_interface.h",
         "src/flatc_main.cpp",
         "src/idl_gen_cpp.cpp",

go/BUILD.bazel

+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go",
+    srcs = [
+        "builder.go",
+        "doc.go",
+        "encode.go",
+        "grpc.go",
+        "lib.go",
+        "sizes.go",
+        "struct.go",
+        "table.go",
+    ],
+    importpath = "github.com/google/flatbuffers/go",
+    visibility = ["//visibility:public"],
+)