BinaryTargetResolver kept a private copy of StableFileHash's descriptor read
loop because it must hash the same descriptor it classified, admitted with
symlinks rejected, while hashStableFile opens its own descriptor and follows
symlinks. The loop is now hashStableRegularFileDescriptor, which takes the
admitted descriptor and the caller's symlink setting; readStableFile and the
resolver both use it. The resolver keeps symlinks: reject and maps a change
to its existing BinaryTargetError and cancellation to AnalysisCancelledError.
