Scan licenses of dependencies

## Short Description
Detect licenses of dependencies too.

## Possible Labels

[dependencies](https://github.com/nexB/scancode-toolkit/labels/dependencies)
[improve-license-detection](https://github.com/nexB/scancode-toolkit/labels/improve-license-detection)

- new feature

## Select Category

- [x ] Enhancement
- [ ] Add License/Copyright
- [ ] Scan Feature
- [ ] Packaging
- [ ] Documentation
- [ ] Expand Support
- [ ] Other

## **Describe the Update**

ScanCode right now looks like it's only scanning for explicit copyright text or license texts in the existing code base. I would like to also be able to pick up licenses of dependencies, and set up flags and where the flagged dependencies have been used in the code base.

For example (this repo for inspiration: https://github.com/raimon49/pip-licenses):
- Set flag to detect GPL licenses
- Pick up all dependencies in my project that uses GPL licenses
- Then spot in my code base where this package was used so I can determine what to do with them etc.

## **How This Feature will help you/your organization**

Identify GPL packages used and where in the code based it was used, so we can detect how we want to handle them, to avoid issues with GPLs.

## **Possible Solution/Implementation Details**

## **Example/Links if Any**

## **Can you help with this Feature**