Refactor fetch and install to be more self-contained

src/fetchers.rs

@@ -0,0 +1,30 @@
+use std::path::Path;
+
+pub use gh_release::*;
+
+use crate::PkgMeta;
+
+mod gh_release;
+
+#[async_trait::async_trait]
+pub trait Fetcher {
+    /// Create a new fetcher from some data
+    async fn new(data: &Data) -> Result<Self, anyhow::Error>
+    where
+        Self: std::marker::Sized;
+
+    /// Fetch a package
+    async fn fetch(&self, dst: &Path) -> Result<(), anyhow::Error>;
+    
+    /// Check if a package is available for download
+    async fn check(&self) -> Result<bool, anyhow::Error>;
+}
+
+/// Data required to fetch a package
+pub struct Data {
+    pub name: String,
+    pub target: String,
+    pub version: String,
+    pub repo: Option<String>,
+    pub meta: PkgMeta,
+}