Build system to report on unused source

It is easy to over-extract source, which is to say, extract source files which are then never used. In theory this is something the build system could spot and report on.

Obviously it requires support in the build framework to make this happen.

No files, tests, or entry points are named. Start by reviewing the build framework and source-extraction flow to determine how extracted files are tracked; done means the build reports extracted source files that are never used.

Written by the indexing model from the issue text.