func: fix use after free on function unload

Functions are stored in lists inside module objects. Module
objects are stored in a hash table, where key is a package name.
But the key was a pointer at one of module's function definition
object. Therefore, when that function was deleted, its freed
package name memory was still in the hash key, and could be
accessed, when another function was deleted.

Now module does not use memory of its functions, and keep a copy
of the package name.

(cherry picked from commit fa2893ea)