Eliminate dependency on serde's "derive" feature

src/assets/assets_metadata.rs

@@ -3,7 +3,7 @@ use std::path::Path;
 use std::time::SystemTime;
 
 use semver::Version;
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 
 use crate::error::*;
 

src/assets/lazy_theme_set.rs

@@ -3,8 +3,7 @@ use super::*;
 use std::collections::BTreeMap;
 use std::convert::TryFrom;
 
-use serde::Deserialize;
-use serde::Serialize;
+use serde_derive::{Deserialize, Serialize};
 
 use once_cell::unsync::OnceCell;