--watchfs should track output files, too

`--watchfs` avoids stating input files in the package path before every build. However, Bazel still checks the validity of cached action state by stating all the action output files it knows about. It would be nice if `--watchfs` could watch output files and obviate stating all of them before execution starts.

Start by tracing how --watchfs avoids stating input files before a build, then compare that flow with the cached action-state validation that states known output files. Done means --watchfs also watches output files so those pre-execution output-file stats are no longer needed.

Written by the indexing model from the issue text.