Search translations directory recursively for localization files

Problem/Motivation

In a composer managed repository, there is a distinction between any files that are provided by packages and those that are local. A typical repository looks as follows:

htdocs (or: web)
-/modules
--/contrib
--/custom
-/themes
--/contrib
--/custom

Drupal can import localization files provided in a local directory or download them from localize.drupal.org. When the webserver is configured to not make any outgoing requests, you can use a composer plugin to download these for you (https://github.com/drupal-composer/drupal-l10n).

It would be nice to have the locale importer search the translations directory, allowing for this directory structure:

translations
-/contrib
-/custom

Proposed resolution

Search translations directory recursively for localization files, so contributed localization files can be saved in a different directory than custom localization files .

Remaining tasks

Write change record

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

Local translations directory can now be organised like modules and theme directories.