Only add the std lib files once (#8830)

# Description

We were seeing duplicate entries for the std lib files, and this PR
addresses that. Each file should now only be added once.

Note: they are still parsed twice because it's hard to recover the
module from the output of `parse` but a bit of clever hacking in a
future PR might be able to do that.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-std/tests/run.nu` to run the tests for the
standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-protocol/src/engine/engine_state.rs

@@ -1345,7 +1345,18 @@ impl<'a> StateWorkingSet<'a> {
         "<unknown>".into()
     }
 
+    #[must_use]
     pub fn add_file(&mut self, filename: String, contents: &[u8]) -> usize {
+        // First, look for the file to see if we already have it
+        for (idx, (fname, file_start, file_end)) in self.files().enumerate() {
+            if fname == &filename {
+                let prev_contents = self.get_span_contents(Span::new(*file_start, *file_end));
+                if prev_contents == contents {
+                    return idx;
+                }
+            }
+        }
+
         let next_span_start = self.next_span_start();
         let next_span_end = next_span_start + contents.len();
 
@@ -1360,6 +1371,15 @@ impl<'a> StateWorkingSet<'a> {
         self.num_files() - 1
     }
 
+    pub fn get_span_for_file(&self, file_id: usize) -> Span {
+        let result = self
+            .files()
+            .nth(file_id)
+            .expect("internal error: could not find source for previously parsed file");
+
+        Span::new(result.1, result.2)
+    }
+
     pub fn get_span_contents(&self, span: Span) -> &[u8] {
         let permanent_end = self.permanent_state.next_span_start();
         if permanent_end <= span.start {
@@ -2113,7 +2133,7 @@ mod engine_state_tests {
 
         let delta = {
             let mut working_set = StateWorkingSet::new(&engine_state);
-            working_set.add_file("child.nu".into(), &[]);
+            let _ = working_set.add_file("child.nu".into(), &[]);
             working_set.render()
         };