QuickInstall support (#94)

See this issue: https://github.com/alsuren/cargo-quickinstall/issues/27

Quick Install is a hosted repo of built crates, essentially. The approach I've taken here is
a list of strategies:

1. First, we check the crate meta or default and build the URL to the repo. Once we have
   that, we perform a `HEAD` request to the URL to see if it's available.
2. If it's not, we build the URL to the quickinstall repo, and perform a `HEAD` to there.

As soon as we've got a hit, we use that. I've built it so it's extensible with more strategies.
This could be useful for #4.

This also adds a prompt before downloading from third-party sources, and logs a short
name for a source, which is easier to glance than a full URL, and includes a quick refactor
of the install/link machinery.

src/fetchers.rs

@@ -0,0 +1,63 @@
+use std::path::Path;
+
+pub use gh_crate_meta::*;
+pub use quickinstall::*;
+
+use crate::{PkgFmt, PkgMeta};
+
+mod gh_crate_meta;
+mod quickinstall;
+
+#[async_trait::async_trait]
+pub trait Fetcher {
+    /// Create a new fetcher from some data
+    async fn new(data: &Data) -> Result<Box<Self>, anyhow::Error>
+    where
+        Self: 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>;
+
+    /// Return the package format
+    fn pkg_fmt(&self) -> PkgFmt;
+
+    /// A short human-readable name or descriptor for the package source
+    fn source_name(&self) -> String;
+
+    /// Should return true if the remote is from a third-party source
+    fn is_third_party(&self) -> bool;
+}
+
+/// Data required to fetch a package
+#[derive(Debug)]
+pub struct Data {
+    pub name: String,
+    pub target: String,
+    pub version: String,
+    pub repo: Option<String>,
+    pub meta: PkgMeta,
+}
+
+#[derive(Default)]
+pub struct MultiFetcher {
+    fetchers: Vec<Box<dyn Fetcher>>,
+}
+
+impl MultiFetcher {
+    pub fn add(&mut self, fetcher: Box<dyn Fetcher>) {
+        self.fetchers.push(fetcher);
+    }
+
+    pub async fn first_available(&self) -> Option<&dyn Fetcher> {
+        for fetcher in &self.fetchers {
+            if fetcher.check().await.unwrap_or(false) {
+                return Some(&**fetcher);
+            }
+        }
+
+        None
+    }
+}