Guess/infer download_url optionally

[pombredanne]

Sometimes, a package will not have a download_url. This is going to be a common case when importing SBOM. We should guess/infer download_url optionally, as well as other information to enrich a package we can get given a PURL.
This could be done through the PurlDB.

[tdruez]

Implemented in #444

• Add "Infer missing download URLs" option on Product importers: "Import SBOM" and "Import Package manifests"
• Add "Infer missing download URLs" option to "Scan all Packages"
• The infer_download_urls is also available in REST API endpoints.

Infer missing download URLs: When a download URL is missing from the input data, attempt to infer it from the Package URL (purl).