Automatic merge from submit-queue

Rename root BUILD to BUILD.bazel, and rename build-tools/ back to build/

**What this PR does / why we need it**: undoes #35453. `build-tools/` was intended to be a temporary workaround until https://github.com/bazelbuild/bazel/issues/552 was fixed, which is has been, as of bazel 0.4.1.

**Which issue this PR fixes**: fixes #38126

FYI @thockin 

since this is likely to be rebase hell, my list of actions:
```console
$ git mv BUILD BUILD.bazel
$ git commit -a
$ git mv build-tools/ build/
$ sift -l 'build-tools' | xargs sed -i -e 's:build-tools/:build/:g'
$ git checkout HEAD docs/
$ git commit -a
```