dev_dependency for bazel_dep

[Wyverald]

Reintroduce dev_dependency to bazel_dep.

We removed it because of potential BUILD breaks in situations like bazelbuild/bazel#12835. However, not having dev dependencies means that the dependency graph could be cluttered in an unbounded manner. For example, most rulesets depend on bazel_skylib, which uses rules_pkg to manage releases. Without dev dependencies, this means that most Bazel projects would depend on bazel_skylib -> rules_pkg -> rules_python -> {insert crapton of deps here}.