Add a buildifier warning to enforce rule load location

[styurin]

Addresses #1231, but without auto fixing.

[styurin]

@vladmos could you take a look, please?

[vladmos]

Is it actually recommended anywhere that genrule should be loaded from "//:genrule.bzl"? Imo it can be confusing for the users who have a different setup in their codebase.

Also, please fix the merge conflict.

[styurin]

Is it actually recommended anywhere that genrule should be loaded from "//:genrule.bzl"? Imo it can be confusing for the users who have a different setup in their codebase.

Updated to use "//tools/bazel:genrule.bzl" instead. This should promote better practices to structure folders with Bazel libraries.

[styurin]

@vladmos could you take another look, please?

[vladmos]

I'm sorry, I still don't understand where the recommendation come from. If that's been discussed e.g. in a mailing list, could you please attach a link? I can imagine that some smaller projects have their own code structure and enforcing a fixed load location without apparent upsides can be noisy. Besides, isn't "genrule" a built-in function that doesn't need to be loaded at all?

[styurin]

There is no such recommendation and it wasn't discussed. There is an issue that describes the problem: #1231

The location of rules is organization-specific and depends on how source code is organized in a given organization. I don't want to provide any recommendations and this PR is not about that. However I'd like to avoid bad examples, so I changed the example location to avoid promoting a practice of keeping bazel rules at the root of a repository.

I'd rather avoid discussing recommendations of any kids in this PR. The purpose of this change it give users a way to enforce their custom rules.