Add new flatbuffer_go_library to generate Go library for flatbuffers

This CL also introduces the following changes to allow the generation of the
Go library for flatbuffers:

- add support for --gen-onefile for Go to simplify the build rule (mapping each
  input .fbs to a single separate .go file)

- add a new --go-import flag to override the default import line (currently
  github.com/google/flatbuffers/go)

- add new go_library in BUILD for flatbuffer (for files in flatbuffers/go)

(mirrored from cr/171126159)

Change-Id: I83e705a9a9d9544837af0baf9366ec37757799aa

src/flatc.cpp

@@ -83,7 +83,7 @@ std::string FlatCompiler::GetUsageString(const char* program_name) const {
       "  --no-includes      Don\'t generate include statements for included\n"
       "                     schemas the generated file depends on (C++).\n"
       "  --gen-mutable      Generate accessors that can mutate buffers in-place.\n"
-      "  --gen-onefile      Generate single output file for C#.\n"
+      "  --gen-onefile      Generate single output file for C# and Go.\n"
       "  --gen-name-strings Generate type name functions for C++.\n"
       "  --gen-object-api   Generate an additional object-based API.\n"
       "  --cpp-ptr-type T   Set object API pointer type (default std::unique_ptr)\n"
@@ -96,6 +96,8 @@ std::string FlatCompiler::GetUsageString(const char* program_name) const {
       "  --no-js-exports    Removes Node.js style export lines in JS.\n"
       "  --goog-js-export   Uses goog.exports* for closure compiler exporting in JS.\n"
       "  --go-namespace     Generate the overrided namespace in Golang.\n"
+      "  --go-import        Generate the overrided import for flatbuffers in Golang.\n"
+      "                     (default is \"github.com/google/flatbuffers/go\")\n"
       "  --raw-binary       Allow binaries without file_indentifier to be read.\n"
       "                     This may crash flatc given a mismatched schema.\n"
       "  --proto            Input is a .proto, translate to .fbs.\n"
@@ -183,6 +185,9 @@ int FlatCompiler::Compile(int argc, const char** argv) {
       } else if(arg == "--go-namespace") {
         if (++argi >= argc) Error("missing golang namespace" + arg, true);
         opts.go_namespace = argv[argi];
+      } else if(arg == "--go-import") {
+        if (++argi >= argc) Error("missing golang import" + arg, true);
+        opts.go_import = argv[argi];
       } else if(arg == "--defaults-json") {
         opts.output_default_scalars_in_json = true;
       } else if (arg == "--unknown-json") {