[antlir2][ci] clean up citadel-generated PACKAGE files

Summary:
These are sitting all over the antlir2/ subdir but they are basically all the
same. They were generated by citadel automation and were not adequately merged
because of contbuild config oddities with how we disable aarch64 for some dirs.

Manually fix these up so that we only have one PACKAGE file at the root and in
a few places where we do need to disable tests for aarch64

Test Plan:
```
❯ buck2 audit package-values fbcode//antlir/antlir2/test_images/rpms
Buck UI: https://www.internalfb.com/buck2/e6fabe3a-f900-4fc8-8d53-4c9e6044cbf5
Network: Up: 0B  Down: 0B
Jobs completed: 2. Time elapsed: 0.0s.
{
  "fbcode//antlir/antlir2/test_images/rpms": {
    "citadel.labels": [
      "ci:linux",
      "ci:linux",
      "ci:linux:opt",
      "ci:linux:aarch64:fbcode//mode/dev-nosan"
    ]
  }
}

❯ buck2 audit package-values fbcode//antlir/antlir2/test_images/rpms/centos8
Buck UI: https://www.internalfb.com/buck2/8c4aab69-29c0-4218-85e8-c723df920670
Network: Up: 0B  Down: 0B
Jobs completed: 2. Time elapsed: 0.0s.
{
  "fbcode//antlir/antlir2/test_images/rpms/centos8": {
    "citadel.labels": [
      "ci:linux",
      "ci:linux",
      "ci:linux:opt"
    ]
  }
}
```

Reviewed By: aniketmathur

Differential Revision: D50601999

fbshipit-source-id: 70d8455f99afc83d2db597e83c2d58a9ca394519

antlir/antlir2/bzl/ci_helpers.bzl

@@ -0,0 +1,18 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+load("@fbcode//target_determinator/macros:ci.bzl", "ci")
+
+def _no_aarch64():
+    parent = ci.package_get()
+    labels = [label for label in parent if "aarch64" not in label]
+    ci.package(
+        labels,
+        overwrite = True,
+    )
+
+ci_helpers = struct(
+    no_aarch64 = _no_aarch64,
+)