Make inference strategy for PURLs a configurable option

[elanzini]

To test the effectiveness of the different heuristics used for the guesswork, the strategy will be separated in 4 different options:

• none : no guessing is performed
• repos : repo URLs are extracted from references of the vulnerability and cross-checked against a cached map that assigns each repo_url to a partial PURL
• cpes : base CPEs provided by NVD are cross-checked against a cached map that matches the base_cpe with a partial PURL.
• both : both strategies are employed

Note: all the maps are build beforehand crawling the respective ecosystems (e.,g. mvn, pypi) and from the CPE dictionary provided by NVD.