Detecting Pillow package

@bndr
I have used pipreqs for various projects of myself or others to identify packages. This package works well in identifying many packages, but in almost all projects work with images and image fields and need Pillow package but this package can not be detected by pipreqs.An enhancement to this project can be adding a feature to have an automatic detection of the Pillow package when there is an image field or function to work with images and pictures in a project.

The issue names no files, tests, or entry points. Start by tracing pipreqs' existing import-detection flow and its test coverage, then define how image-related usage should identify Pillow and add coverage for the requested case. Done means generated requirements include Pillow for the described project.

Written by the indexing model from the issue text.