language/go: infer rules_go version from WORKSPACE (#905)

We prefer to read RULES_GO_VERSION from @io_bazel_rules_go//go:def.bzl,
but that won't be available if the bazel-out symlink hasn't been
created yet.

As a fallback, look for the io_bazel_rules_go declaration in
WORKSPACE, and try to extract the version from the URLS.

Fixes #780