Returns empty packages information when trying to scan a .whl package

### Description
I am trying to scan a `.whl` package to get all the package info from it (such as package name, version etc...) but the result gives always empty list in packages section (Not giving any package related info such as name, version, description etc...). same for `jars` aswell
```
{
"headers": [
{
"tool_name": "scancode-toolkit",
"tool_version": "3.0.2",
"options": {
"input": "python_repos/pip_downloads_from_requirements/jmespath-0.9.4-py2.py3-none-any.whl",
"--info": true,
"--json-pp": "scancode_res.json",
"--package": true,
"--processes": "2"
},
"notice": "Generated with ScanCode and provided on an \"AS IS\" BASIS, WITHOUT WARRANTIES\nOR CONDITIONS OF ANY KIND, either express or implied. No content created from\nScanCode should be considered or used as legal advice. Consult an Attorney\nfor any legal advice.\nScanCode is a free software code scanning tool from nexB Inc. and others.\nVisit https://github.com/nexB/scancode-toolkit/ for support and download.",
"start_timestamp": "2019-04-01T150032.328119",
"end_timestamp": "2019-04-01T150035.541706",
"message": null,
"errors": [],
"extra_data": {
"files_count": 1
}
}
],
"files": [
{
"path": "jmespath-0.9.4-py2.py3-none-any.whl",
"type": "file",
"name": "jmespath-0.9.4-py2.py3-none-any.whl",
"base_name": "jmespath-0.9.4-py2.py3-none-any",
"extension": ".whl",
"size": 24161,
"date": "2019-04-01",
"sha1": "4e18f8857463b944b861448eccfeec4edcf09cd3",
"md5": "730e8863a8c03bae9d93234c77b39aef",
"mime_type": "application/zip",
"file_type": "Zip archive data, at least v2.0 to extract",
"programming_language": null,
"is_binary": true,
"is_text": false,
"is_archive": true,
"is_media": false,
"is_source": false,
"is_script": false,
"packages": [],
"files_count": 0,
"dirs_count": 0,
"size_count": 0,
"scan_errors": []
}
]
}
```
Help on same is much appreciated !

### System configuration
For bug reports, it really helps us to know:
* What OS are you running on? Linux
* What version of scancode-toolkit was used to generate the scan file? -> 3.0.2
* What installation method was used to install/run scancode? pip