Separate off IO from nargo command logic

[TomAFrench]

Problem

Currently we've got IO logic spread across the entire cli directory - some in a X_cmd.rs file to be used locally, some in mod.rs to be used in several files.

This causes a large amount of noise which makes it harder to find the actual business logic.

Solution

We can create single module which holds all of the IO logic which the files defining each command can just import as needed.

auth
├─ mod.rs
├─ commands
│  ├─ check_cmd.rs
│  ├─ compile_cmd.rs 
│  ├─ etc.
├─ fs
│  ├─ keys.rs
│  ├─ inputs.rs
│  ├─ etc.

The major benefit I see for this is that it makes reworking how we perform IO (necessary for the new build artifact format) easier due to the increased encapsulation.

Other benefits include:

• simplifying the CLI files.
• making it easier to define a specific error type for IO issues.

Alternatives considered

We can maintain the IO alongside the CLI logic.

Additional context