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/project_panel/src/project_panel.rs

@@ -644,7 +644,7 @@ mod tests {
         });
         let (root1, _) = project
             .update(&mut cx, |project, cx| {
-                project.find_or_create_worktree_for_abs_path("/root1", false, cx)
+                project.find_or_create_local_worktree("/root1", false, cx)
             })
             .await
             .unwrap();
@@ -653,7 +653,7 @@ mod tests {
             .await;
         let (root2, _) = project
             .update(&mut cx, |project, cx| {
-                project.find_or_create_worktree_for_abs_path("/root2", false, cx)
+                project.find_or_create_local_worktree("/root2", false, cx)
             })
             .await
             .unwrap();