[JavaScript] モジュールのインポートに失敗する

聞きたいこと

モジュールのインポートをする箇所で、下のようなエラーが発生してしまい解決できません。。
モジュールインポートの際にパスで記述する際のエラーのようですが、、、絶対パス等で入力しても解決せず・・・

エラー内容

Uncaught TypeError: Failed to resolve module specifier "SpriteSheet". Relative references must start with either "/", "./", or "../".

ディレクトリ構成

:
- public
  |- js
     |- main.js
     |- SpriteSheet.js
     |- loaders.js
  |- img 
:

main.js

javascript
1import SpriteSheet from './SpriteSheet';
2import {loadImage, loadLevel} from './loaders';
3
4function drawBackground(background, context, sprites) {
5  background.ranges.forEach(([x1, x2, y1, y2]) => {
6    for (let x = x1; x < x2; ++x) {
7      for (let y = y1; y < y2; ++y) {
8        sprites.drawTile(background.tile, context, x, y);
9      }
10    }
11  });
12}

SpriteSheet.js

export default class SpriteSheet {
  constructor (image, width, height) {
    this.image = image;
    this.width = width;
    this.height = height;
    this.tiles = new Map();
  }

  define(name, x, y) {
    const buffer = document.createElement('canvas');
    buffer.width = this.width;
    buffer.height = this.height;
    buffer
          .getContext('2d')
          .drawImage(
            x * this.width,
            y * this.height,
            this.width,
            this.height,
            0,
            0,
            this.width,
            this,height);
    this.tiles.set(name, buffer);
  }

  draw(name, context, x, y) {
    const buffer = this.tiles.get(name);
    context.drawImage(buffer, x, y);
  }
  
  drawTile(name, context, x, y) {
    this.draw(name, context, x * this.width, y * this.height);
  }
}

loaders.js

export function loadImage(url) {
  return new Promise(resolve => {
    const image = new Image();
    image.addEventListener('load', () => {
      resolve(image);
    });
    image.src = url;
  });
}

export function loadLevel(name) {
  return fetch(`/levels${name}.json`)
         .then(r => r.json());
}

package.json

{
  "name": "super-mario",
  "version": "1.0.0",
  "description": "",
  "main": "main.js",
  "scripts": {
    "start": "serve ./public",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "serve": "^10.0.2"
  }
}