[structure view] more fine-grained check for existing catalog files when adding default ones

[martinlippert]

At the moment, we add the default catalog files when there are absolutely no catalog files found in the project or on the classpath. This makes it unnecessary cumbersome for users when adding their own custom stereotypes via catalog files, since adding a custom catalog file into the project automatically disables the default spring and jpa catalog files from being taken into account. The workaround is to add dependencies to the related jmolecules artifacts that contain the spring and jpa catalog files, but that us cumbersome and most likely not what you want (add extra stuff to the classpath only to make the tools work).

Instead, we should add the default catalog files (spring, jpa, and the basic java ones as well) in cases where the corresponding artifacts from jmolecules are not in the classpath.