Refer to third party dependencies using project relative path

This has the advantage of making `go build` and `go test` work without a
wrapper script (e.g. `script/build`) for any platform, hence making build process
easier. A direct consequence is Windows user can run `go build` to build
without us providing an extra shell script.

I manually fixed all import paths this time but I'll provide a script later.

/cc @mislav