Address feedback from PR #12229 (#12242)

# Description
@sholderbach left a very helpful review and this just implements the
suggestions he made.

Didn't notice any difference in performance, but there could potentially
be for a long running Nushell session or one that loads a lot of stuff.

I also caught a bug where nu-protocol won't build without `plugin`
because of the previous conditional import. Oops. Fixed.

# User-Facing Changes
`blocks` and `modules` type in `EngineState` changed again. Shouldn't
affect plugins or anything else though really

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

---------

Co-authored-by: sholderbach <sholderbach@users.noreply.github.com>

crates/nu-cli/src/completions/completer.rs

@@ -124,7 +124,7 @@ impl NuCompleter {
 
         let output = parse(&mut working_set, Some("completer"), line.as_bytes(), false);
 
-        for pipeline in output.pipelines.iter() {
+        for pipeline in &output.pipelines {
             for pipeline_element in &pipeline.elements {
                 let flattened = flatten_pipeline_element(&working_set, pipeline_element);
                 let mut spans: Vec<String> = vec![];