Maintain preprocessors/backends execution order as defined in book.toml

There's a regression caused by recent refactor work, as it used to execute
preprocessors/backends in order as defined in book.toml, but now this is
not the case, which causes trouble when some backends depend on the result
from another backend (e.g. mdbook-pdf). The root cause is that a HashMap
has no order, so this PR switch this into BTreeMap instead.

Signed-off-by: Hollow Man <[email protected]>

Author: HollowMan6

crates/mdbook-core/src/config.rs

@@ -48,7 +48,7 @@ use crate::utils::log_backtrace;
 use anyhow::{Context, Error, Result, bail};
 use log::{debug, trace};
 use serde::{Deserialize, Serialize};
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 use std::env;
 use std::fs::File;
 use std::io::Read;
@@ -214,15 +214,15 @@ impl Config {
     }
 
     /// Returns the configuration for all preprocessors.
-    pub fn preprocessors<'de, T: Deserialize<'de>>(&self) -> Result<HashMap<String, T>> {
+    pub fn preprocessors<'de, T: Deserialize<'de>>(&self) -> Result<BTreeMap<String, T>> {
         self.preprocessor
             .clone()
             .try_into()
             .with_context(|| "Failed to read preprocessors")
     }
 
     /// Returns the configuration for all renderers.
-    pub fn outputs<'de, T: Deserialize<'de>>(&self) -> Result<HashMap<String, T>> {
+    pub fn outputs<'de, T: Deserialize<'de>>(&self) -> Result<BTreeMap<String, T>> {
         self.output
             .clone()
             .try_into()