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-explore/src/nu_common/command.rs

@@ -91,6 +91,8 @@ fn eval_source2(
     // So we LITERALLY ignore all expressions except the LAST.
     if block.len() > 1 {
         let range = ..block.pipelines.len() - 1;
+        // Note: `make_mut` will mutate `&mut block: &mut Arc<Block>`
+        // for the outer fn scope `eval_block`
         Arc::make_mut(&mut block).pipelines.drain(range);
     }