Surface warnings in the syft JSON document

Today the syft JSON document shows positive findings of packages. It would be ideal if it additionally enumerated areas that were skipped or otherwise partially processed. In https://github.com/anchore/syft/pull/348 we adjusted the cataloger to be more lenient and https://github.com/anchore/syft/issues/359 was created to ensure the remaining catalogers are consistent in this manner. This issue aims to explore surfacing warnings (as described in #348 ) directly in the JSON document to enable machine-readable awareness of these issues. A potential alternative to this is to have the caller enable structured logging and capture all warning and error log level events, however, this is potentially less ideal.

Start by reviewing PR #348 and issue #359 to understand the cataloger warnings and the consistency concerns they describe. Determine how warnings could be represented in the syft JSON document, then compare that approach with structured logging; done means callers can machine-read skipped or partially processed areas.

Written by the indexing model from the issue text.