Handling `go_repository` rules

[bweston92]

When you use go_repository rules in your WORKSPACE file to declare dependencies.

How do we get the dependencies pulled to go into the GOPATH created by gobazel?

[linuxerwang]

gobazel would not copy source code to GOPATH, instead, it maps source code with FUSE. You can think of it as kind of symlinks. Once all source code files are mapped (with the rules we configured), the Go compiler can find the deps correctly.