Move 'PackageName' and associated methods in its own module.

This is a bit cleaner, as the 'cmd/new' had many on-the-fly functions
  which are better scoped inside this module.

  Plus, it plays nicely with the std::str::FromStr trait definition.

crates/aiken-project/Cargo.toml

@@ -18,6 +18,7 @@ hex = "0.4.3"
 ignore = "0.4.18"
 itertools = "0.10.1"
 miette = { version = "5.3.0", features = ["fancy"] }
+owo-colors = "3.5.0"
 pallas = "0.16.0"
 pallas-traverse = "0.16.0"
 petgraph = "0.6.2"