* Refactor: Extract new fn `BinFile::check_source_exists`
* Impl new async fn `AutoAbortJoinHandle::flattened_join`
* Impl new fn `Fetcher::fetcher_name`
* Verify that `bin_files` exist in `resolve` stage
To ensure that the installation stage won't fail because of missing
binaries.
* Rm unused `MultiFecther`
* Simplify `Future` impl for `AutoAbortJoinHandle`
* Add new variant `BinstallError::CargoTomlMissingPackage`
* Replace `unwrap` in `resolve_inner` with proper error handling
* Make `Fetcher::new` as a regular function
instead of an `async` function.
* Ret `Arc<dyn Fetcher>` in trait fn `Fetcher::new`
* Refactor `resolve_inner`
Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>
