Harden installer translation file download against potential infinite redirects

Problem/Motivation

This is a follow-up to 🐛 InstallerTranslationExistingFileTest fails on 11.x branch Active . In that issue, we fixed a test that started failing when a new translation file was added to the translation server. We discovered that the installer code that decides what file name to download during install was able to select and download a file that the FileTranslation scanner can't find. This caused an infinite loop where the installer requests a translation file, downloads it, reloads the page in the new language, and the FileTranslation service doesn't detect a translation file, and orders it downloaded again, restarting the process.

When the initial version change was committed, the translation file that the installer was requesting didn't exist. In this case, tests passed for a time as the installer recognized it couldn't find the file, and redirected to an error page (The test is only checking for a 200 response after downloading the file). It wasn't until a week or two later that the file the installer was looking for (which the translator couldn't detect) was added to the translation server, that tests started failing. The issue was tricky to discover because it wasn't obviously tied to any specific commit when looking through test runs on d.o, and in fact builds that had passed on d.o. previously were now failing if re-ran.

We fixed the immediate issue with the specific file name that was causing problems. This follow-up is to consider hardening the system to prevent future potential issues, particularly those that are dependent on which files exist on the translation server.

Steps to reproduce

Proposed resolution

From the original issue:

I'm wondering if we should do something more robust here, like add a query parameter on the redirect indicating that we downloaded the translation file, so when we go back through we see that we thought we downloaded it, but can't find it and report the error instead of an endless redirect. Or a more structured way of ensuring that whatever is set as the drupal version will pass the FileTranslation scanner.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet