Infer purl from download URL

It would be great if we could infer a purl using heuristics based on a URL and/or filename and/or filepath.

These heuristics are package-type specific.

For instance with http://repo1.maven.org/maven2/io/github/subiyacryolite/jds/3.0.1/jds-3.0.1-sources.jar we could get pkg:maven/io.github.subiyacryolite/jds@3.0.1?qualifier=sources&type=jar .

Or from https://api.nuget.org/v3-flatcontainer/newtonsoft.json/10.0.1/newtonsoft.json.10.0.1.nupkg we could infer pkg:nuget/newtonsoft.json@10.0.1

Or a path with github.com/foo/bar (such as from a vendored Go package) could yield `pkg:golang/github.com/foo/bar` and so on.

Since these would be heuristics and we cannot get 100% success we should return a confidence level with an inferred purl.