Support scanning Composer Stager library for translatable strings

Problem/Motivation

The Composer Stager library is being developed for Drupal. See 📌 Add php-tuf/composer-stager to core dependencies and governance — for experimental Automatic Updates & Project Browser modules Needs work . Because it will remain a separate Composer library, it will not be automatically scanned for translatable strings the way Drupal core is--but it still needs to be. See Add support for string translation for names, descriptions, and messages · Issue #123 · php-tuf/composer-stager. This issue is to discuss adding support for it to potx.

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Proposed resolution

• Add vendor/php-tuf/composer-stager to scanned locations.
• Add pattern matching according to the outcome of Add support for string translation for names, descriptions, and messages · Issue #123 · php-tuf/composer-stager

Remaining tasks

TBD

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API changes

???

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