Collect and filter cabal data in build plans

There are a number of places (excluding custom setups, certain licenses, etc) where explicitly disallowing certain values of certain fields from solver build plans could be useful, or, alternately, warning on such values, or simply providing reporting of the union of such values.

While third party tools can make use of generated info after the fact to report on build plans, it would be very good to provide cabal with a general purpose way to filter the index presented to the solver on any top level cabal field (either positive or negative presence of certain values) as well as report on the union of data of any cabal field, so that one could see all licenses in a plan and their provenance, or all uses of custom setups in a plan and their provenance, etc.

Start by tracing how Cabal presents the package index to the solver and how solver build plans are generated. Review the existing generated-info and third-party reporting paths mentioned in the issue, then define the supported filtering and union-reporting scope. Done means the behavior is specified for top-level Cabal fields, including provenance for reported values.

Written by the indexing model from the issue text.