[ticket/12963] Revert back to "migrations" folder name for extensions

The issues that can be created with the name change are just too much

PHPBB3-12963
[ticket/12963] Remove old migration/ folder from tests

PHPBB3-12963
[ticket/12963] Always use core_path and core_directory()

PHPBB3-12963
Merge pull request #2967 from nickvergessen/ticket/12963

[ticket/12963] Revert back to "migrations" folder name for extensions

* nickvergessen/ticket/12963:
  [ticket/12963] Always use core_path and core_directory()
  [ticket/12963] Remove old migration/ folder from tests
  [ticket/12963] Revert back to "migrations" folder name for extensions