[ticket/12051] Optimise Composer Autoloader on Build

PHPBB3-12051
Merge pull request #2644 from Nicofuma/ticket/12051

[ticket/12051] Optimise Composer Autoloader on Build

* Nicofuma/ticket/12051:
  [ticket/12051] Optimise Composer Autoloader on Build
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12051] Optimise Composer Autoloader on Build