Rename and refine top level attributes to align with SCTK

- `headers` should be a list of mappings, not just a mapping to align with SCTK structures
- `requirements` could be renamed to `dependencies` because this is exactly how this is already named in ScanCode Toolkit
- in `resolved_dependencies`:
- `package` should be renamed to `purl` for consistency
- `wheel_urls` and `sdist_url` need some refinements:
- they could be merged in a single list and the attribute renamed to `download_urls`
- they could be renamed to `binary_download_urls` or `package_download_urls` for the wheels and `source_download_urls` (plural) for the sdist. This is in preparation of having other inspectors that should use the same data structures.
- the sub `dependencies` attributes may need a bit more thinking, as this may be best understood as `resolved_dependencies` (e.g. reusing the same name and having some sorts of nested data structure)