idempotent symlink
idempotent symlink

The official interface doc for buildpacks[1] doesn't say
that buildpacks always run in a fresh container or
anything of the like, so we ought to assume that things
we did in previous invocations might stick around.

[1]: https://devcenter.heroku.com/articles/buildpack-api

heroku/heroku-buildpack-go,stevenwilkin/go-buildpack,Sing-Li/go-buildpack,hpcloud/go-buildpack,hpcloud/go-buildpack,virtmerlin/go-buildpack,half-ogre/go-buildpack,venicegeo/go-pdal-buildpack,Jonty/go-buildpack,cloudfoundry/go-buildpack,enkoder/heroku-buildpack-go,Sing-Li/go-buildpack,loafoe/go-buildpack,stevenwilkin/go-buildpack,loafoe/go-buildpack,Jonty/go-buildpack,virtmerlin/go-buildpack,Sing-Li/go-buildpack,cloudfoundry/go-buildpack,michael-go/heroku-buildpack-go,heroku/heroku-buildpack-go,venicegeo/go-pdal-buildpack,hpcloud/go-buildpack,michael-go/heroku-buildpack-go,venicegeo/go-pdal-buildpack,Sing-Li/go-buildpack,half-ogre/go-buildpack,Sing-Li/go-buildpack,Sing-Li/go-buildpack,cloudfoundry/go-buildpack,Sing-Li/go-buildpack,virtmerlin/go-buildpack,stevenwilkin/go-buildpack,loafoe/go-buildpack,titanous/heroku-buildpack-go,half-ogre/go-buildpack,Jonty/go-buildpack,Sing-Li/go-buildpack