Rename 'project_core' crate to 'worktree', make it just about worktrees (#9189)

This is just a refactor. I noticed that we now have a `project_core`
crate, which mainly contains the `Worktree` type and its private
helpers, plus the project's settings.

In this PR, I've renamed that crate to `worktree` and did some minor
simplification to its module structure. I also extracted a new
`WorktreeSettings` settings type from the `ProjectSettings`, so that the
worktree settings could live in the worktree crate. This way, the crate
is now exclusively about worktree logic.

Release Notes:

- N/A

crates/extension/Cargo.toml

@@ -10,6 +10,7 @@ workspace = true
 
 [lib]
 path = "src/extension_store.rs"
+doctest = false
 
 [[bin]]
 name = "extension_json_schemas"

crates/extension/src/build_extension.rs

@@ -260,13 +260,6 @@ impl ExtensionBuilder {
             .args(["fetch", "--depth", "1", "origin", &rev])
             .output()
             .context("failed to execute `git fetch`")?;
-        if !fetch_output.status.success() {
-            bail!(
-                "failed to fetch revision {} in directory '{}'",
-                rev,
-                directory.display()
-            );
-        }
 
         let checkout_output = Command::new("git")
             .arg("--git-dir")
@@ -276,6 +269,13 @@ impl ExtensionBuilder {
             .output()
             .context("failed to execute `git checkout`")?;
         if !checkout_output.status.success() {
+            if !fetch_output.status.success() {
+                bail!(
+                    "failed to fetch revision {} in directory '{}'",
+                    rev,
+                    directory.display()
+                );
+            }
             bail!(
                 "failed to checkout revision {} in directory '{}'",
                 rev,