Allow modules to incentivize packets instead of assuming it is a user account

[crodriguezvega]

This would require:

• changing SendCoinsFromAccountToModule to SendCoins.
• possibly changing PacketFee.RefundAddress to sdk.AccAddress.
• changing bank calls in distributeFee.

[AdityaSripal]

I think this is worth doing given that one use case that came up in discussions around this feature was a module paying for fees on their chain. This will entail changing any call SendCoinsFromModuleToAccount and SendCoinsFromAccountToModule to be SendCoins.

We will need to add check in each case that the receiver of the coins is not a blocked address.

cc: @colin-axner

[colin-axner]

To check if we need to adjust the incentivizing address to be a sdk.AccAddress, try to write a test which uses a module to incentivize the packet

[seantking]

Spoke with @AdityaSripal & @colin-axner after writing a test. It turns out this already works out of the box:

This function already does what we were planning on doing:
https://github.com/cosmos/cosmos-sdk/blob/b6478026c4e8c5f03c7e6c6f726a60debb9bbfac/x/bank/keeper/keeper.go#L318