Deprecate --os

[deadlysyn]

Deprecate --os in favor of --package-manager. Minor cleanup along the way.

This pull request makes the following changes:

• Deprecate --os (shouldn't break builds thanks to auto detection)
• Add --package-manager
• Minor updates to purl format for consistency across formats
• Minor tweak to flag setup in chase.go inspired by iq.go
• Update tests to use --package-manager

(If there are changes to user behavior in general, please make sure to
update the docs, as well)

It relates to the following issue #s:

• Addresses Cleanup references to os #29

cc @bhamail / @DarthHater / @zendern

[sonatypecla]

Thanks for the contribution! Before we can merge this, we need @deadlysyn to sign the Sonatype Contributor License Agreement.

[deadlysyn]

Sorry for the noise, I submitted too soon -- going to close and re-open next week. :-(