Modify generate_purls to better handle missing versions.

[JosiahParry]

When a package is missing a version OSS Index is searched for all package versions. This can lead to false positive vulnerability reports. This PR adds a warning in the event of missing package versions

This pull request makes the following changes:

• Adds check to the internal function generate_purls() to identify missing package versions
• The function now generates a message in the console via `cli_() functions if packages are missing versions

There should be no breaking changes or changes to existing behavior.

cc @bhamail / @DarthHater / @brittanybelle / @adrianpowell / @csgillespie

[csgillespie]

Thanks @JosiahParry

Just merged it into dev and I'll tidy up. You've fallen back to your bad ways and started using <- ;)

I'm also not sure what * does in sonatype, so I've pinged @DarthHater