Use HashMap<ProjectEntryId, Entry> instead of HashSet<Entry> in outline_panel (#20780)

Came across this because I noticed that `Entry` implements `Hash`, which
was surprising to me. I believe that `ProjectEntryId` should be unique
and so it seems better to dedupe based on this.

Release Notes:

- N/A

crates/worktree/src/worktree.rs

@@ -3344,7 +3344,7 @@ impl File {
     }
 }
 
-#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub struct Entry {
     pub id: ProjectEntryId,
     pub kind: EntryKind,
@@ -3376,7 +3376,7 @@ pub struct Entry {
     pub is_fifo: bool,
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum EntryKind {
     UnloadedDir,
     PendingDir,