Import Unmanaged Solution as a Managed Solution

I've exported an unmanaged solution as a managed solution and now trying to import it back into the system. While doing so, it gives me an error 

Name: ImportMissingDependenciesError

Hex: 8004801D

All the dependent files which it says missing are in the system but not in the solution. I'm confused if to import an unmanaged solution as a managed solution, all the dependent files have to be a part of a solution or not?

( I did an experiment to check if I can export this solution as an unmanaged solution and import it back into the system without including all the dependent files into the solution and surprisingly it did get imported so I'm not sure why is it happening?)

Please, any suggestion.