Allow port redefinition from an empty port only

[ducky64]

To unify the syntax with how ports are usually defined (self.io = self.Port(DigitalBidir(models))) with when a base class defines an io (self.io = self.Port(DigitalBidir.empty())), instead of using .init_from, support an implicit .init_from using redeclaration syntax.

It is still an error to redeclare a port in any other context, including a partially-initialized port.

Related: to better support IO re-exporting, perhaps there needs to be a Block.export_ios_from, which (asserting the IOs exist in the block already) connects the internal to the external, matching by name.

• It is an error if there is an internal IO that does not have an external counterpart, but it is not an error if there are extra external IOs
• IOs still must be explicitly instantiated, to allow for static checking. One pattern might be to define an MyClassIo(Block) base class that only instantiates empty IOs of the right type.
• for Connector-pairs for daughterboards #367, similar construct for WrapperBlock.export_tap_ios_from for the export_tap variant.

Related: #114 compositional passive