Use a single source dir for the drivers and drivers-initfs recipes

This makes it easier to make driver changes. And in the future other
recipes could potentially benefit from the same infrastructure too.

recipes/core/drivers-initfs/recipe.toml

@@ -1,5 +1,5 @@
 [source]
-git = "https://gitlab.redox-os.org/redox-os/drivers.git"
+same_as = "../drivers"
 
 [build]
 template = "custom"

src/bin/cook.rs

@@ -162,6 +162,25 @@ fn run_command_stdin(mut command: process::Command, stdin_data: &[u8]) -> Result
 fn fetch(recipe_dir: &Path, source: &Option<SourceRecipe>) -> Result<PathBuf, String> {
     let source_dir = recipe_dir.join("source");
     match source {
+        Some(SourceRecipe::SameAs { same_as }) => {
+            if ! source_dir.is_symlink() {
+                if source_dir.is_dir() {
+                    return Err(format!(
+                        "'{dir}' is a directory, but recipe indicated a symlink. \n\
+                        try removing '{dir}' if you haven't made any changes that would be lost",
+                        dir=source_dir.display(),
+                    ));
+                }
+                let original = Path::new(same_as).join("source");
+                std::os::unix::fs::symlink(&original, &source_dir).map_err(|err| format!(
+                    "failed to symlink '{}' to '{}': {}\n{:?}",
+                    original.display(),
+                    source_dir.display(),
+                    err,
+                    err
+                ))?;
+            }
+        }
         Some(SourceRecipe::Git { git, upstream, branch, rev }) => {
             //TODO: use libgit?
             if ! source_dir.is_dir() {

src/recipe.rs

@@ -4,6 +4,14 @@ use serde::{Deserialize, Serialize};
 #[derive(Debug, Deserialize, PartialEq, Serialize)]
 #[serde(untagged)]
 pub enum SourceRecipe {
+    /// Reuse the source directory of another package
+    ///
+    /// This is useful when a single source repo contains multiple projects which each have their
+    /// own recipe to build them.
+    SameAs {
+        /// Relative path to the package for which to reuse the source dir
+        same_as: String,
+    },
     /// A git repository source
     Git {
         /// The URL for the git repository, such as https://gitlab.redox-os.org/redox-os/ion.git