pkgset: Only load cache once

The file in old compose does not change, there is no need to load it
again for each tag.

One consequence of this change is that the same cache will be used by
all package sets. They add stuff to the cache as they use it. However
that should not be a problem for the same reason it's okay to use the
cache in the first place. If two packages have the same path, they are
actually the same file and it's okay to reuse the data.

JIRA: COMPOSE-3374
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/phases/pkgset/pkgsets.py

@@ -236,12 +236,17 @@ class PackageSetBase(kobo.log.LoggingBase):
                         rpm_path = rpm_path[len(remove_path_prefix):]
                     f.write("%s\n" % rpm_path)
 
-    def load_old_file_cache(self, file_path):
+    @staticmethod
+    def load_old_file_cache(file_path):
         """
         Loads the cached FileCache stored in pickle format in `file_path`.
         """
         with open(file_path, "rb") as f:
-            self.old_file_cache = pickle.load(f)
+            return pickle.load(f)
+
+    def set_old_file_cache(self, old_file_cache):
+        """Set cache of old files."""
+        self.old_file_cache = old_file_cache
 
     def save_file_cache(self, file_path):
         """