Get rid of mod.rs files.

Since Rust 2018 [1] it is no longer required to move module code to
`mod.rs` when submodules are added. This eliminates common problem of
having multiple buffers named `mod.rs` in editor.

[1] https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html#no-more-modrs

src/sql.rs

@@ -755,7 +755,7 @@ mod test {
         let t = TestContext::new().await;
 
         let avatar_src = t.dir.path().join("avatar.png");
-        let avatar_bytes = include_bytes!("../../test-data/image/avatar64x64.png");
+        let avatar_bytes = include_bytes!("../test-data/image/avatar64x64.png");
         File::create(&avatar_src)
             .await
             .unwrap()