Add PURL support to VulnDB analyzer

VulnDB now supports PURL and has a dedicated endpoint for making queries.

See: https://vulndb.cyberriskanalytics.com/doc/classes/Api/V1/VulnerabilitiesController.html#method-i-find_by_purl

Additionally, VulnDB mirroring should include PURLs so that the Internal Analyzer can make use of them.

Note: It appears that VulnDB PURLs conform to the spec, but there are some issues. Support ticket opened.