Simplify kapitan/inputs/base.py

[gburiola]

Suggested by @MatteoVoges on #979 (comment)

Not sure why, but kapitan/inputs/base.py iterates on input_paths and not on the comp_obj directly.

With the changes introduced on #979, not all compile objects will have input_paths.

A workaround was introduced here to deal with these specific compile objects without input_paths

Aim of this issue is to investigate why we iterate on input_paths and not directly on comp_obj. If possible / feasible, change code to iterate on comp_obj to simplify the code.

@ramaro may have more context on this.

[ramaro]

This might be a historic, original decision that just stayed like this while other things continued to change. I will give this some thought.

[github-actions]

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