Provide feedback to user when an exact manifest digest was not provided

(mirror of https://github.com/anchore/grype/issues/485 to request this same enhancement within _Syft_ — let's centralize design discussion and conclusions in this **Grype issue** before following up on this Syft issue)

Start with the linked Grype issue, where the description says the design discussion and conclusions should be centralized. Review those conclusions before determining the corresponding Syft change; the work is done when Syft provides feedback when an exact manifest digest was not provided.

Written by the indexing model from the issue text.