Detect unecessary work

Fab should be able to identify operations it performs which do not contribute to the build. For example source which is extracted but never used. Source which is processed but then never compiled.

Start by tracing how Fab records or executes build operations, focusing on source extraction, processing, and compilation. Define how the system can determine whether each operation contributes to the final build; done means unnecessary operations are identified reliably, with coverage for unused extracted and processed-but-uncompiled source.

Written by the indexing model from the issue text.