[antlir2][package] add explicit rule for caf packages

Summary:
This does nothing but return the layer's `subvol_symlink`, but allows us to rip
some CAF packaging hacks out of the `fbpkg.builder` implementation.

This eliminates bugs in `image_fbpkg_builder` where `default_os` configuration
was not being applied for CAF packages.

I will leave it to justintrudell to remove any hacks from the builder rule.

Test Plan:
Hope that
`fbcode//registry/builder/fbpkg/tests:test-caf-image-builder-e2e` runs on
Sandcastle because I cannot run it on my devserver

Reviewed By: epilatow

Differential Revision: D59007520

fbshipit-source-id: b65f4d4a72402a0b762c431ff9cfaf0de9a13d33

antlir/antlir2/bzl/package/caf.bzl

@@ -0,0 +1,25 @@
+# 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("//antlir/antlir2/bzl:types.bzl", "LayerInfo")
+load(":cfg.bzl", "layer_attrs", "package_cfg")
+load(":macro.bzl", "package_macro")
+
+CafLayerPackageInfo = provider(fields = {"subvol_symlink": Artifact})
+
+def _impl(ctx: AnalysisContext) -> list[Provider]:
+    li = ctx.attrs.layer[LayerInfo]
+    return [
+        ctx.attrs.layer[DefaultInfo],
+        CafLayerPackageInfo(subvol_symlink = li.subvol_symlink),
+    ]
+
+_caf = rule(
+    impl = _impl,
+    attrs = layer_attrs,
+    cfg = package_cfg,
+)
+
+caf = package_macro(_caf)