Refactor default output directory

[ferraith]

If the user doesn't pass a default or model specific output path the package source directory will be used as a fallback. This behavior should be changed for two reasons:

• In case multiple equally named Ecore models are generated the output path isn't unique. pyecoregen will overwrite the generated code
• Intuitively the user will expect that the generated code is placed in parallel to the Ecore XMI file and not in the root folder of the package.