## Summary

* Move code that handles package managing to its own crate named `pacquet_package_manager`.
  * Convert functions with multiple arguments into structs.
* Make `pacquet add` reuses the algorithm from `pacquet install`.
  * Achieved by having `pacquet add` creates an in-memory `package.json` and pass it to the `Install` subroutine.
* Convert the error-prone tests in `crates/cli` into proper integration tests that invoke the `pacquet` binary.
  * Tests become shorter and more readable.
  * Solve race condition issues with `cargo test`.
* All code with `set_current_dir` have been replaced by either integration tests or dependency injection.
  * Solve race condition issues with `cargo test`.
* `thiserror` has been completely removed. `derive_more` is used in its stead.
  * `thiserror` cannot handle generic trait bound.
  * `derive_more` is already used to generate custom string types in the lockfile.
  * one less macro crate leads to slightly faster compile time.
* `pacquet_package_json::PackageJson` has been renamed to `pacquet_package_manifest::PackageManifest`.
* Other minor changes.