[ticket/12508] Remove extension manager from finder

PHPBB3-12508
[ticket/12508] Fix usages of the finder

PHPBB3-12508
[ticket/12508] Move \phpbb\extension\finder to \phpbb\finder

PHPBB3-12508
[ticket/12508] Fix class doc block

PHPBB3-12508
[ticket/12508] Only take a list of names for set_extensions()

PHPBB3-12508
[ticket/12508] Add a unit test for set_extensions()

PHPBB3-12508
[ticket/12508] Add new line to separate the if and foreach better

PHPBB3-12508
[ticket/12508] Fix doc block

PHPBB3-12508
[ticket/12508] Ignore extensions in migration_tips dev tool

PHPBB3-12508
Merge remote-tracking branch 'github-nickvergessen/ticket/12508' into develop-ascraeus

* github-nickvergessen/ticket/12508:
  [ticket/12508] Ignore extensions in migration_tips dev tool
  [ticket/12508] Fix doc block
  [ticket/12508] Add new line to separate the if and foreach better
  [ticket/12508] Add a unit test for set_extensions()
  [ticket/12508] Only take a list of names for set_extensions()
  [ticket/12508] Fix class doc block
  [ticket/12508] Move \phpbb\extension\finder to \phpbb\finder
  [ticket/12508] Fix usages of the finder
  [ticket/12508] Remove extension manager from finder