Add batch support for localpacks package parsing

Good idea.

However, I'd suggest not applying the same limit to the batch process like we do for the cron runs. The cron run is just one request and the batch process is as many requests as many packages we want to process, so we can have a much higher limit for the batch compared to the cron runs. Look at how I did a very similar solution in connectors/l10n_project.

Also, I'd suggest not to use the form submit function as an API function but instead just have an API function and reuse that from the form submit handler. Faking the form submission in the cron hook seems like a bad idea.

Finally, there are all kinds of whitespace issues. The start of l10n_localpacks_check_files() lacks whitespace, the l10n_localpacks_scan_batch_build() function has excessive whitespace and the phpdoc comments seem to only be stubs.

Do we still still need that now that 6.x is discontinued?