feat(rm)!: use arg. spans for I/O errors (#8964)

# Description

Currently, error spans for I/O errors in an `rm` invocation always point
to the `rm` argument. This isn't ideal, because the user loses context
as to which “target” actually had a problem:


![image](https://user-images.githubusercontent.com/658538/235723366-50db727e-9ba2-4d16-afc6-6a2406c584e0.png)

Shadow the existing `span` variable in outer scope in `rm`'s
implementation for the errors that may be detected while handling I/O
results. This is desired, because all failures from this point are
target-specific, and pointing at the argument that generated the target
instead is better. The end user should now see this:


![image](https://user-images.githubusercontent.com/658538/235724345-1d2e98e0-6b20-4bf5-b8a2-8b4368cdfb05.png)

# User-Facing Changes
* When `rm` encounters I/O errors, their spans now point to the “target”
argument associated with the error, rather than the `rm` token.

# Tests + Formatting


No tests currently cover this. I'm open to adding tests, but adding as
follow-up sounds better ATM, since this wasn't covered before.

# After Submitting

Nothing needs to be done here, AFAIK. No I/O errors are currently
demonstrated in official docs, though maybe they should be?

crates/nu-command/src/filesystem/rm.rs

@@ -318,8 +318,8 @@ fn rm(
     }
 
     all_targets
-        .into_keys()
-        .map(move |f| {
+        .into_iter()
+        .map(move |(f, span)| {
             let is_empty = || match f.read_dir() {
                 Ok(mut p) => p.next().is_none(),
                 Err(_) => false,