Define droidcore and droidcore-unbundled in sooong-only builds

Some modules register disted files against these phonies, and
then they get disted when running `m droid dist` because they're
dependencies of droid in make. Recreate this behavior in soong-only
builds.

Bug: 388312357
Test: m --soong-only dist dists acloud and aidegen
Change-Id: I9f2300967752ce1564db44a552d41144ff40cffd
diff --git a/packaging/main_soong_only.mk b/packaging/main_soong_only.mk
index 6d17314..2c61e10 100644
--- a/packaging/main_soong_only.mk
+++ b/packaging/main_soong_only.mk
@@ -31,11 +31,17 @@
 droid: droid_targets
 
 .PHONY: droid_targets
-droid_targets: dist_files
+droid_targets: droidcore dist_files
 
 .PHONY: dist_files
 dist_files:
 
+.PHONY: droidcore
+droidcore: droidcore-unbundled
+
+.PHONY: droidcore-unbundled
+droidcore-unbundled:
+
 $(info [2/4] including distdir.mk ...)
 
 include build/make/packaging/distdir.mk