Fix go_repository configuration (#559)

* go_repository now supports a build_config attribute. By default,
  this points to "@//:WORKSPACE", but it may reference another file
  that changes less frequently or None. This is used to set the
  -repo_config option when invoking Gazelle.
* A global default may be set via "go_repository_default_config" in
  gazelle_dependencies.
* go_repository now reads files with "cat" instead of ctx.read.

Fixes #555