Improve readability for apache_httpd importer

See line 87 above in the importer class, there is a lot of dict/key usage, often several levels like:
`for vendor in data["affects"]["vendor"]["vendor_data"]` and then again `for products in vendor["product"]["product_data"]`.

Shouldn't we probably use something like the `AdvisoryData` classes (or a special subclass for that) which would model the advisory data directly, and then use those classes everywhere instead of dicts. This has some advantages:

1. code would be more pythonic and readable (and better looking)
2. we can and should test for the advisory format and handle keyerrors gracefully (idk if the data format changes that much at all, but it could in future?)

_Originally posted by @AyanSinhaMahapatra in https://github.com/nexB/vulnerablecode/pull/1102#discussion_r1088817834_