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-protocol/src/ast/expression.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use serde::{Deserialize, Serialize};
 
-use super::{Argument, Expr, ExternalArgument, RecordItem};
+use super::{Argument, Block, Expr, ExternalArgument, RecordItem};
 use crate::ast::ImportPattern;
 use crate::DeclId;
 use crate::{engine::StateWorkingSet, BlockId, Signature, Span, Type, VarId, IN_VARIABLE_ID};
@@ -327,7 +327,8 @@ impl Expression {
                 expr.replace_span(working_set, replaced, new_span);
             }
             Expr::Block(block_id) => {
-                let mut block = (**working_set.get_block(*block_id)).clone();
+                // We are cloning the Block itself, rather than the Arc around it.
+                let mut block = Block::clone(working_set.get_block(*block_id));
 
                 for pipeline in block.pipelines.iter_mut() {
                     for element in pipeline.elements.iter_mut() {