[antlir2][migration] make btrfs.new respect migration mode

Summary:
This was still using antlir1, but worked because it did all kinds of sendstream
nonsense internally.

Now it'll respect the `PACKAGE` file settings

Test Plan:
```
❯ buck2 build --show-output fbcode//tupperware/image/virt:virt.fog_supervisor.btrfs
Buck UI: https://www.internalfb.com/buck2/7bc77940-6d81-4d58-8fab-db00039b0e62
Network: Up: 15MiB  Down: 1.7GiB  (reSessionID-7eaa66bd-cf6b-4623-a08a-7b04bd0ba425)
Jobs completed: 278811. Time elapsed: 3:01.1s.
Cache hits: 95%. Commands: 1104 (cached: 1045, remote: 0, local: 59). Fallback: 10/59
BUILD SUCCEEDED
fbcode//tupperware/image/virt:virt.fog_supervisor.btrfs buck-out/v2/gen/fbcode/a4c48ac5654ab817/tupperware/image/virt/__virt.fog_supervisor.btrfs__/image.btrfs

vmagro@devvm11640.ftw0 in fbsource
❯ sudo mount buck-out/v2/gen/fbcode/a4c48ac5654ab817/tupperware/image/virt/__virt.fog_supervisor.btrfs__/image.btrfs /var/tmp/btrfs/mnt

vmagro@devvm11640.ftw0 in fbsource
❯ ls /var/tmp/btrfs/mnt
volume

vmagro@devvm11640.ftw0 in fbsource
❯ ls /var/tmp/btrfs/mnt/volume
bin  boot  data  dev  etc  home  lib  lib64  media  mnt  opt  packages  proc  root  run  sbin  srv  sys  tmp  usr  var
```

Reviewed By: justintrudell

Differential Revision: D52564943

fbshipit-source-id: e0a0ee3d10769727f4bf3a60194fd6c134d3c4d2

antlir/antlir2/bzl/package/btrfs.bzl

@@ -63,9 +63,10 @@ _btrfs = rule(
         "compression_level": attrs.int(default = 3),
         # used by transition
         "default_os": attrs.option(attrs.string(), default = None),
-        "default_subvol": attrs.option(attrs.string()),
+        "default_subvol": attrs.option(attrs.string(), default = None),
         "free_mb": attrs.option(attrs.int(), default = None),
         "label": attrs.option(attrs.string(), default = None),
+        "labels": attrs.list(attrs.string(), default = []),
         "seed_device": attrs.bool(default = False),
         "subvols": attrs.option(
             attrs.dict(

antlir/bzl/image/package/btrfs.bzl

@@ -4,6 +4,8 @@
 # LICENSE file in the root directory of this source tree.
 
 load("@bazel_skylib//lib:shell.bzl", "shell")
+load("//antlir/antlir2/bzl/package:btrfs.bzl?v2_only", antlir2_BtrfsSubvol = "BtrfsSubvol", antlir2_btrfs = "btrfs")
+load("//antlir/bzl:antlir2_shim.bzl", "antlir2_shim")
 load("//antlir/bzl:bash.bzl", "wrap_bash_build_in_common_boilerplate")
 load("//antlir/bzl:build_defs.bzl", "buck_genrule")
 load("//antlir/bzl:loopback_opts.bzl", "normalize_loopback_opts")
@@ -101,7 +103,50 @@ def _new_btrfs(
         antlir_rule = antlir_rule,
     )
 
+def _new_btrfs_shim(
+        name,
+        # Opts are required
+        opts,
+        # Buck `labels` to add to the resulting target; aka `tags` in fbcode.
+        labels = None,
+        visibility = None,
+        antlir_rule = "user-facing"):
+    if opts.loopback_opts.size_mb:
+        fail("size_mb not supported in btrfs")
+    if opts.loopback_opts.fat_size:
+        fail("fat_size not supported in btrfs")
+    opts_kwargs = {
+        "compression_level": opts.compression_level,
+        "default_subvol": opts.default_subvol,
+        "free_mb": opts.free_mb,
+        "label": opts.loopback_opts.label,
+        "labels": labels,
+        "seed_device": opts.seed_device,
+        "subvols": {
+            subvol_name: antlir2_BtrfsSubvol(
+                layer = subvol.layer,
+                writable = subvol.writable,
+            )
+            for subvol_name, subvol in opts.subvols.items()
+        },
+    }
+
+    if antlir2_shim.upgrade_or_shadow_package(
+        antlir2 = None,
+        name = name,
+        fn = antlir2_btrfs,
+        visibility = visibility,
+        fake_buck1 = struct(
+            fn = antlir2_shim.fake_buck1_target,
+            name = name,
+        ),
+        **opts_kwargs
+    ) == "upgrade":
+        return
+
+    _new_btrfs(name, opts, labels, visibility, antlir_rule)
+
 btrfs = struct(
-    new = _new_btrfs,
+    new = _new_btrfs_shim,
     opts = _btrfs_opts_api,
 )