Previously, we were using 8.0.0rc1.
In particular, this upgrade means we need to explicitly
import more rules, as they've been moved out of the core bazel repo.
* fixed 5.0.0 as bazel version
* made dependencies better loadable
* moved `//swift/install` to `//swift:create-extractor-pack` (following
the clearer ruby naming)
* renamed `extractor_pack` to `extractor-pack` for consistency with Ruby