Remove Router from IBCKeeper and use PortKeeper

[damiannolan]

Summary

We currently maintain a reference to the IBC application router on the IBCKeeper as well as the PortKeeper submodule keeper.
This is often referred to as the "port router" as this is how we look up application callback stacks in order to route application handshake msgs and packet data to the appropriate handlers.

With moving towards a more consistent usage of standardising submodule keepers as reference types (see #6081) we should now be able to remove the Router reference from the IBCKeeper and instead rely directly on the one in PortKeeper.

With this change we can also probably make Router unexported within PortKeeper and add a Route() method on the PortKeeper instead, and all application callback stack access can flow through there.

• Remove Router from IBCKeeper
• Add GetRoute() types.IBCModule to PortKeeper
• Replace usage of Router in core/keeper/msg_server.go with PortKeeper.Route()

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[bznein]

@damiannolan I wouldn't mind taking this one :)

[chatton]

go for it @bznein this one depends on #6081 to me merged first