Align package name with maxs-logger

Currently, the rexs-api package name is `info.rexs.*`, while the one for the maxs-logger is `de.fva_net.*`.
As both packages are developed by the FVA GmbH and carry their copyright notice at the top of the files, I propose that we align the package names to `de.fva_net.*` everywhere. This should be applied both to the maven package name as well as the java package paths.