Package layout: directories, __init__.py, nested packages and private (_name) module variables

A package is how Python groups related modules into a folder hierarchy. The directory name becomes the package name, each .py file inside is a module, and each subdirectory can be a subpackage. Traditionally, and in everything the PCAP course teaches, each package directory contains a file called __init__.py, which marks the directory as a regular package. Python 3 can also import directories without it (namespace packages), but for the exam treat __init__.py as the package marker.