Make gazelle consider only BUILD.bazel files

### What does this PR do?
Configure the `//:gazelle` target with `-build_file_name=BUILD.bazel`.

### Motivation
When `gazelle` runs inside a Linux container on macOS via Docker
Desktop, the workspace is typically mounted on a case-insensitive
filesystem where `BUILD` and `build` are indistinguishable.

This causes spurious errors such as:
```sh
bazel run //:gazelle; echo $?
...
gazelle: open /dda/.gitlab/windows/build: no such file or directory
open /dda/rtloader/build: no such file or directory
1
```

Whereas the cause strictly resides in a filesystem layer/driver
(docker/desktop-feedback#251, recently imported from docker/for-mac#7218),
portable tools have been taking countermeasures. Bazel 9, for instance,
revamped their filesystem cache to better accomodate case-insensitive
filesystems (bazelbuild/bazel#26842), which led us to switch to it
(#47742), but that only covers Bazel's own I/O, not the processes it
spawns, of which `gazelle`.

For that purpose, Gazelle happens to explicitly recommend setting
its `build_file_name` directive to `BUILD.bazel`:
> If a repository contains files named `build` that aren't related to Bazel, it may help to set this to `"BUILD.bazel"`, especially on case-insensitive file systems.

### Additional Notes
For consistency with the narrowed scope, this change renames
`compliance/rules/BUILD` to `compliance/rules/BUILD.bazel` (the only
plain `BUILD` file still present in the tree).

Author: rdesgroppes

BUILD.bazel

@@ -446,7 +446,10 @@ ALL_BUILD_TAGS = [
 # gazelle:prefix github.com/DataDog/datadog-agent
 gazelle(
     name = "gazelle",
-    args = ["-external=static"],  # don't use the network: https://github.com/bazel-contrib/bazel-gazelle/issues/1385
+    args = [
+        "-build_file_name=BUILD.bazel",  # avoid BUILD/build mess: https://github.com/docker/desktop-feedback/issues/251
+        "-external=static",  # don't use the network: https://github.com/bazel-contrib/bazel-gazelle/issues/1385
+    ],
     build_tags = ALL_BUILD_TAGS,
     gazelle = ":gazelle_binary",
 )