Minimal PSR-3 compatible logger layer
Provide a minimal PSR-3 compatible logger layer to make backporting
critical updates from 1.25+ easier. It also serves to allow extensions
that maintain backwards compatibility with the 1.23 LTS releases to
migrate to the new logging system.
A copy of the official PSR-3 implementation is added in
includes/libs/psr3 with the omission of PHP5.4+ Traits. In MediaWiki
1.25+ this library will be provided via Composer.