User-defined PSR4-namespace directory overrides existing

https://gitlab.com/hpierce1102/ClassFinder/-/blame/master/src/PSR4/PSR4NamespaceFactory.php#L26

If my project defines a namespace mbolli\project (e.g. /src) and a vendored package defines the same one (e.g. /vendor/mbolli/projectDeps/src), the two are not merged. The first project takes precedence, the vendored one is ignored. IMHO this is an issue -> array_merge should be array_merge_recursive.