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