[META] Porting localization server module to Drupal 9

Problem/Motivation

Localize is still using D7, we need to migrate it to D9. One of the main blocker for this is the l10n_server module. This issue focuses on porting this module to D9.

Proposed resolution

There are currently 3 different contexts, using each a dedicated connector:

File upload (eg drupal.8.7.po): l10n_gettext
Folder containing tar.gz files: l10n_drupal
Pulling a .tsv file from drupal.org: l10n_drupal_rest (this is the one used on localize.drupal.org)

The core feature that we want to focus on at the moment is the l10n_drupal_rest connector, as it is the only one coming from drupal.org and it allows Localize to get the full list of existing projects (https://www.drupal.org/files/releases.tsv).

There is a strong dependency to l10n_community within the current code, but its features are specific to localize.drupal.org: it displays the latest Drupal core translation status and data. So we agreed that there is no need to require it within l10n_server for now, we will port the code without these features - they should probably live in a dedicated localize module.

Development setup

We have a D9 platform branch here to simplify the team work.
Please see the README: https://git.drupalcode.org/issue/localizedrupalorg-3273538/-/tree/327353...

Remaining tasks

• #3292736: Porting l10_server submodule
• #3280685: Porting l10n_drupal_rest
• #3282696: Port of l10n_community
• #3274238: Port l10n_packager
• #3292197: Port l10n_groups / #3395640: Test l10n groups implementation
• #3292192: Port localization server testsuite to Drupal 9
• #3292220: Create data migration or upgrade path
• #3292198: Port l10n_remote / #3292824: Provide a REST based API
• #3311393: [META] UX/UI thinking / #3311393: [META] UX/UI thinking
• #3312295: Implement class resolver pattern in the .module file
• #3312293: Move the current code repository to the new localize repository
• #3312291: Update documentation for the local environment setup

Others:

• Port hook_menu (non blocker, l10n_community dependency) and linked dashboard > WIP by FMB
• Finish creating l10n_stats submodule
• #2447765: [D8] - Port Bluecheese to D8
• #3292239: Different string count for D7 / D9 versions of potx