refactor(lib_core): task manager structure

refs: #5

cli/bin/readwise/external_interface.rs

@@ -1,4 +1,4 @@
-use lib_sync_core::task_manager::TaskPayload;
+use lib_sync_core::tasks::TaskPayload;
 use chrono::{DateTime, Local};
 use serde::{de, Deserialize, Deserializer, Serialize};
 use serde_json::Value;

cli/bin/readwise/main.rs

@@ -4,7 +4,7 @@ use figment::{
     Figment,
     providers::{Env, Serialized},
 };
-use lib_sync_core::task_manager::{TaskManager, TaskStatus};
+use lib_sync_core::tasks::{TaskStatus};
 use cli::config::{Command, Config};
 use cli::{Error, Result};
 use std::fs::File;