fix: avoid including duplicate import map if it's imported from program

[magurotuna]

This commit ensures that one import map appears just once in the generated eszip.

Fixes #110

Now a generated eszip works fine with eszip_builder and eszip_viewer.

eszip_builder

$ cargo run --example eszip_builder file://$(pwd)/main.js out.eszip file://$(pwd)/import_map.json
source: file:///Users/yusuktan/Repo/github.com/magurotuna/eszip/import_map.json
source: file:///Users/yusuktan/Repo/github.com/magurotuna/eszip/main.js

eszip_viewer

$ cargo run --example eszip_viewer out.eszip
Specifier: file:///Users/yusuktan/Repo/github.com/magurotuna/eszip/import_map.json
Kind: Json
---
{
  "imports": {
  }
}

---

============
Specifier: file:///Users/yusuktan/Repo/github.com/magurotuna/eszip/main.js
Kind: JavaScript
---
import m from "./import_map.json" assert { type: "json" };

---

============

[satyarohith]

LGTM - nice test!