[assembly] Resolve issue of cache packages also being listed in base

Remove any pkgs in the 'cache' set that are already in the 'base' set,
so that they only exist in one or the other location inside the
generated assembly config.

Change-Id: Ib630e9d4114977d63ac37a4c8db21ababc4c4d1b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/638624
Fuchsia-Auto-Submit: Aaron Wood <[email protected]>
Reviewed-by: James Robinson <[email protected]>
Reviewed-by: James Tucker <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

Author: woody77

build/images/assembly/make_assembly_config.py

@@ -39,7 +39,13 @@ def main():
 
     if args.cache_packages_list is not None:
         cache_packages_list = json.load(args.cache_packages_list)
-        config["cache"] = cache_packages_list
+
+        # Strip all base pkgs from the cache pkgs set, so there are no
+        # duplicates across the two sets.
+        cache_packages_set = set(cache_packages_list)
+        cache_packages_set.difference_update(config["base"])
+
+        config["cache"] = list(sorted(cache_packages_set))
 
     extra_packages = []
     if args.extra_files_packages_list is not None: