The log file config is located in /var/lib/openhab2/etc/org.ops4j.pax.logging.cfg.
You can usually guess at the truncated part of the binding because they are all in the same packages for the most part.
org.openhab.binding.<binding name>
Usually the “binding.<binding name>” part gets preserved.
There are a few bindings that will be in the org.eclipse.smarthome.binding.<binding name> path but that should be changing over the coming weeks or months if it hasn’t already.
Since you are familiar with log4j, you are aware that you can specify the format of the log lines through the logger config file so that is one way to get the full package name.
Usually what I’ll do is I’ll configure a separate logger and appender for the binding I am debugging and configure it to log at debug or trace level to it’s own file.
A good example can be found at Log4j2 Sample Config