Thank you a lot!
@kristoffer.carlsson Is it “save” to use Base.loaded_modules with respect to future Julia 1.x releases? From the answer to this question Using an already-loaded module in 0.7 - #6 by StefanKarpinski I assumed I should avoid using it.
@dmolina you are right, I was talking about packages. This package helps already a lot. I want to figure out what pacakges (and versions) are loaded in a custom system image. At leaste, for my own packages I am able to do that now, thanks.