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/workspace/src/workspace.rs

@@ -723,7 +723,7 @@ impl Workspace {
         cx: &mut ViewContext<Self>,
     ) -> Task<Result<ProjectPath>> {
         let entry = self.project().update(cx, |project, cx| {
-            project.find_or_create_worktree_for_abs_path(abs_path, false, cx)
+            project.find_or_create_local_worktree(abs_path, false, cx)
         });
         cx.spawn(|_, cx| async move {
             let (worktree, path) = entry.await?;