patch 8.0.0612: pack dirs are added to 'runtimepath' too late

Problem:    Package directories are added to 'runtimepath' only after loading
            non-package plugins.
Solution:   Split off the code to add package directories to 'runtimepath'.
            (Ingo Karkat, closes #1680)
diff --git a/src/globals.h b/src/globals.h
index 7bc5471..56df6d5 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -326,6 +326,8 @@
 EXTERN scid_T	current_SID INIT(= 0);
 #endif
 
+EXTERN int	did_source_packages INIT(= FALSE);
+
 /* Magic number used for hashitem "hi_key" value indicating a deleted item.
  * Only the address is used. */
 EXTERN char_u	hash_removed;