Split the code into a library

[mssola]

The code of zypper-docker should be split into a library that will be used by the final binary. The reasoning for this is:

• This way we establish a clearer scope of the code and it will be easier to maintain.
• This way we can re-use the library part in other components that might be developed in the future (e.g. a serve command).

See the existing cli-separation branch.