Always use custom logger, when available.

Blacklight's built-in logger will now check if a custom logger has been subsequently loaded via plugin. If so, then the built-in logger will use the custom logger instead.

This is a workaround for a kind of race condition, where some modules are initialized with BL’s built-in logger before the custom logger has been found and configured.

Without this fix, these early-loaded modules will only ever log using BL’s built in logger.