A great plugin by Philipp Salvisberg for SQL Developer tool for unwrapping PL/SQL code can be found here: https://www.salvis.com/blog/2015/05/17/introducing-plsql-unwrapper-for-sql-developer/
I used this to inspect some of the internal Apex packages, and it works like a charm for 10g and above. For 9i and earlier I have not found an out-of-the-box solution yet.
The first time I tried the free service Unwrap it!
I got an error, so I gave up on that at once.
In the past I have used Niels Teusink’s Python script unwrap.py
and that worked fine. I do prefer a GUI however, so although I am not a fan of SQL Developer, it is more convenient for this type of thing.