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/src/deps/downloader.rs

@@ -4,8 +4,8 @@ use futures::future;
 use reqwest::Client;
 
 use crate::{
-    config::PackageName,
     error::Error,
+    package_name::PackageName,
     paths::{self, CacheKey},
 };
 

crates/aiken-project/src/deps/manifest.rs

@@ -5,8 +5,9 @@ use miette::NamedSource;
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    config::{Config, Dependency, PackageName, Platform},
+    config::{Config, Dependency, Platform},
     error::Error,
+    package_name::PackageName,
     paths,
     telemetry::{Event, EventListener},
 };