Reorder wallet imports

[Neotamandua]

#2947 for rusk-wallet, test-wallet & wallet-core

I'm intentionally leaving all pub use, mod and other exports as they are, because it's still up for discussion how we want to order them.

[moCello]

LGTM

Two general comments but none blocking:

1. Do we want to separate the workspace deps from the external deps or mix them in one block?
2. I think the rusk-wallet deps in the rusk-wallet binary should be separated from the external deps

[Neotamandua]

LGTM

Two general comments but none blocking:

1. Do we want to separate the workspace deps from the external deps or mix them in one block?

2. I think the `rusk-wallet` deps in the `rusk-wallet` binary should be separated from the external deps

I would prefer not to separate them further. Even if they are logically tied, they are still external dependencies at some point. (Hovering over rusk_wallet in main.rs for example tells you extern crate rusk_wallet)

For imports, I would only distinguish between std, external, and crate. And to me, anything that is external is something that cannot be referenced by self, super, or crate.