Set the default value for 'packpath'

As noted in “:help 'packpath'”, the default value is supposed to be the
same as that for 'runtimepath'.  This was missed in the original port of
the packages functionality from Vim.

Closes #5193

src/nvim/option.c

@@ -521,6 +521,8 @@ static void set_runtimepath_default(void)
 #undef SITE_SIZE
 #undef AFTER_SIZE
   set_string_default("runtimepath", rtp, true);
+  // Make a copy of 'rtp' for 'packpath'
+  set_string_default("packpath", rtp, false);
   xfree(data_dirs);
   xfree(config_dirs);
   xfree(data_home);