Add the `weaknesses` data in the API `packages` endpoint.

[tdruez]

https://public.vulnerablecode.io/api/vulnerabilities/7567

...
"weaknesses": [
        {
            "cwe_id": 352,
            "name": "Cross-Site Request Forgery (CSRF)",
            "description": "The web application does not, or can not, sufficiently verify whether a well-formed, valid, consistent request was intentionally provided by the user who submitted the request."
        },
        {
            "cwe_id": 1035,
            "name": "OWASP Top Ten 2017 Category A9 - Using Components with Known Vulnerabilities",
            "description": "Weaknesses in this category are related to the A9 category in the OWASP Top Ten 2017."
        },
        {
            "cwe_id": 937,
            "name": "OWASP Top Ten 2013 Category A9 - Using Components with Known Vulnerabilities",
            "description": "Weaknesses in this category are related to the A9 category in the OWASP Top Ten 2013."
        }
    ],
...

This is essential data to collect but it is missing from the affected_by_vulnerabilities data structure.
For example https://public.vulnerablecode.io/api/packages/156170

Make sure to add the proper QuerySet optimization (prefetch_related).

[Mukund-Menon]

Opened a PR for this. PTAL!