storage: Simplify layout of storage crate (servo#40124)

Due to old design choices, we had the layout of:
```
| - storage
|    - indexeddb
|        - mod.rs
|        - idb_thread.rs
|        - engines/
|    - webstorage
|        - mod.rs
|        - webstorage_thread.rs
|        - engines/
```

I merged the `*_thread` file into `mod.rs`, since `mod.rs` is pretty
empty. This should be a better layout for when more threads are
introduced.

Testing: Refactor, none needed

Signed-off-by: Ashwin Naren <[email protected]>

Author: arihant2math

components/storage/indexeddb/engines/sqlite.rs

@@ -18,8 +18,8 @@ use storage_traits::indexeddb_thread::{
 };
 use tokio::sync::oneshot;
 
+use crate::indexeddb::IndexedDBDescription;
 use crate::indexeddb::engines::{KvsEngine, KvsTransaction};
-use crate::indexeddb::idb_thread::IndexedDBDescription;
 use crate::shared::{DB_INIT_PRAGMAS, DB_PRAGMAS};
 
 mod create;