What would you like to be added:
Today we support scanning a set of purls from a newline delimited file, but we don't support scanning a single purl from the CLI without the need to involve a file. A current workaround to this is:

cat pkg:apk/mypurl@1.2.3 | grype purl:/dev/stdin

But more ideally we should be able to support this with:

grype pkg:apk/mypurl@1.2.3

Why is this needed:
This makes it really easy to scan one-off packages without the need to get docker or other files involved.