Make add_local_worktree private and use find_or_create_local_worktree

The former always adds a worktree, even if we have one already in the
project and that could be misused. The public API should always search
for a local worktree containing the requested path first so that the
project can uphold invariants about which worktrees it has.

crates/diagnostics/src/diagnostics.rs

@@ -765,7 +765,7 @@ mod tests {
 
         project
             .update(&mut cx, |project, cx| {
-                project.add_local_worktree("/test", false, cx)
+                project.find_or_create_local_worktree("/test", false, cx)
             })
             .await
             .unwrap();