engine: Fix up some send/recv corner cases

Initially I wasn't 100% clear or decided on the send/recv semantics.
After some experimenting, I think this is much closer to what we want.
Nothing should break or regress here, this only enables more
possibilities.

engine/resources/file.go

@@ -1373,6 +1373,7 @@ func (obj *FileRes) CheckApply(ctx context.Context, apply bool) (bool, error) {
 	// might not have a new value to copy, and therefore we won't see this
 	// notification of change. Therefore, it is important to process these
 	// promptly, if they must not be lost, such as for cache invalidation.
+	// NOTE: Modern send/recv doesn't really have this limitation anymore.
 	if val, exists := obj.init.Recv()["content"]; exists && val.Changed {
 		// if we received on Content, and it changed, invalidate the cache!
 		obj.init.Logf("contentCheckApply: invalidating sha256sum of `content`")