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-std/src/lib.rs

@@ -7,12 +7,10 @@ fn add_file(
     name: &String,
     content: &[u8],
 ) -> (Module, Vec<Span>) {
-    let start = working_set.next_span_start();
-    working_set.add_file(name.clone(), content);
-    let end = working_set.next_span_start();
+    let file_id = working_set.add_file(name.clone(), content);
+    let new_span = working_set.get_span_for_file(file_id);
 
-    let (_, module, comments) =
-        parse_module_block(working_set, Span::new(start, end), name.as_bytes());
+    let (_, module, comments) = parse_module_block(working_set, new_span, name.as_bytes());
 
     if let Some(err) = working_set.parse_errors.first() {
         report_error(working_set, err);