Gazelle required when using //:protoc-gen-validate with Bazel

[mjduijn]

When using one of the rules in //bazel:pgv_proto_library.bzl from another workspace you need to specify Gazelle as a dependency (in that workspace). While I realize a lot of people using Bazel automatically import Gazelle, it's really only useful on go and proto projects.

Because the rules use //:protoc-gen-validate and in that same file the following load statement occurs: load("@bazel_gazelle//:def.bzl", "gazelle"). (I think) this can be prevented by moving the //:protoc-gen-validate to another place, for instance, inside the module directory.