Export FLOWS and build from the top-level fluksio package

A test of the declarations had to import them from fluksio.sdk while the
guide teaches `from fluksio import ...`. The worker's inert module gets the
same two names so a module importing them still loads inside a node.

worker/fluksio_worker/worker_main.py

@@ -101,6 +101,9 @@ class _Reporter(ModuleType):
     #: The same name the SDK exports at top level, so a node written against
     #: one surface finds it on the other. Set by ``_install_reporter``.
     logger: logging.Logger
+    #: The registry `fluksio sync` reads, empty here: nothing registers into
+    #: it inside a node. Set by ``_install_reporter``.
+    FLOWS: dict[str, Any]
 
     def emit(self, **ports: Any) -> None:
         """Publish on this node's output ports without returning yet.
@@ -174,6 +177,10 @@ class _Reporter(ModuleType):
         """A flow declaration, already read by `fluksio sync`."""
         return _Declared()
 
+    def build(self, *args: Any, **kwargs: Any) -> Any:
+        """A flow document, already built by `fluksio sync`."""
+        return {}
+
 
 class _LogHandler(logging.Handler):
     """Writes to whatever ``sys.stderr`` is at the time of the record.
@@ -409,6 +416,7 @@ def _install_reporter() -> None:
     # its own logging installs a root handler on the same stream.
     logger.propagate = False
     module.logger = logger
+    module.FLOWS = {}
     sys.modules["fluksio"] = module