Add support for binary package references

### Description
We already track the sources packages of a package. We would need to track the inverse relationship which would be the "binary" or "built" packages from a "source" package (such as a Git repo or an upstream tarball).

See https://github.com/nexB/scancode-toolkit/blob/450205506989607c8361b17e1bce0db63c9923b2/src/packagedcode/models.py#L465

This would be a new attribute with this definition:
```
binary_packages = List(
item_type=String,
label='List of related binary packages',
help='A list of related binary (or built or derived) Package URLs (aka. "purl") for '
'this package. For instance an RPM archive or an npm tarball is the "binary package" for a '
'binary SRPM or upstream GitHub repository.')
```