The package is a compiled version of your code where the java code is compiled to a jar files and is included in the /apps/<sitename>/install folder along with ui.apps content. Rest of the configurations under /apps, etc can be imported into eclipse, but not the java code.
If you want to develop again, then you need the source code of the application which you can build again into package. Adobe provides a standard https://github.com/adobe/aem-project-archetype which defines the structure of the project with its best practices.
If you aim is to just debug, you can use any decompiler to extract the /apps/<sitename>/install/<name>.jar file to see the java classes.