Fix hot reloading for virtual files

[guybedford]

I've been using a virtual filesystem in the new JSPM Sandbox with:

      self.esmsInitOptions = {
        onpolyfill:() => {},
        hotReload: true,
        source:(url, fetchOpts, parentUrl, defaultSourceHook) => {
          if (!url.startsWith('file:///virtual-pkg/')) return defaultSourceHook(url, fetchOpts, parent);
          const versionQueryParam = url.match(/\\?v=\\d+$/);
          if (versionQueryParam) url = url.slice(0, -versionQueryParam[0].length);
          let source = window.parent.jspmFileSystem[url.slice('file:///virtual-pkg/'.length)];
          if (!source) throw new Error(\`Virtual module \${url} not found, imported from \${parent}\`);
          source += '\\n//# sourceURL=about:blank\\n';
          switch (url.slice(url.lastIndexOf('.') + 1)) {
            case 'mjs': case 'js': return { type: 'js', source };
            case 'wasm': return { type: 'wasm', source };
            case 'css': return { type: 'css', source };
            case 'json': return { type: 'json', source };
            case 'tsm': case 'ts': return { type: 'ts', source };
          }
        },
        onerror:e=>window.parent.jspmSandboxError(e.message || e, '', '', '', e)
      };

where the file system is loaded virtually through the source hook.

This works great, and even supports CSS hot reloading, but the JS hot reloading on the virtual file system scheme file:///virtual-pkg/foo.js doesn't seem to reload properly.

It would be great to get some proper tests together for the virtual sandboxing use cases further to ensure this works.