複数の.swiftファイルをSwift Package Managerを用いずにコンパイルする ref: https://qiita.com/YutoMizutani/items/ba8f993ef2ef8b04fb18

file2.txt

$ swiftc main.swift hello.swift
$ ./main

file3.sh

cat << 'EOT' > main.swift
import Foundation

let hello = Hello()
hello.say()
EOT

cat << 'EOT' > hello.swift
import Foundation

struct Hello {
    func say() {
        print("Hello, world!!")
    }
}
EOT

swiftc -o main main.swift hello.swift
./main
rm main.swift hello.swift main  # 後片付け

hello.swift

import Foundation

struct Hello {
    func say() {
        print("Hello, world!!")
    }
}

main.swift

import Foundation

let hello = Hello()
hello.say()