In Lisp it’s quite common to have your own helper libraries, which are packaged as ASDF packages. To be able to use your libraries in other projects, you need to let ASDF know how to find them.
First create the directory ~/.config/common-lisp/source-registry.conf.d/
There create a file with any name of your choice but with the type conf, for instance 50-petar-lisp.conf.
In this file, add the following line to tell ASDF to recursively scan all the subdirectories under /home/petar/lisp/ for .asd files: (:tree "/home/petar/lisp/").