Settings files_exists check.

[dpagini]

Changes proposed:

• Adds a check for docroot/sites/$multisite/default.settings.php exists before it tries to copy that file to docroot/sites/$multisite/settings.php

Reasoning...
In my multisite, I like to keep a single copy of Drupal's default.settings.php file. We keep it in docroot/sites/all/ and have it copied there via composer and drupal-scaffold (see below). I think this means we will be responsible for generating a new settings file if we set up a new multi-site, but I think that's a fair trade-off. This really just stops this function from throwing an error for me that the file does not exist.
Maybe in a perfect world, this could optionally read the drupal-scaffold setting, if it exists, from composer.json, and then use that value, but that felt a little too optimistic to me. Figured I'd see where this one goes first.

in composer.json

drupal-scaffold: {
  initial: {
    sites/default/default.services.yml: "sites/default/services.yml",
    sites/default/default.settings.php: "sites/default/settings.php"
  },
  excludes: [
    "web.config",
    "sites/development.services.yml"
  ]
}

[grasmash]

I think you want $copy_map[$project_default_settings_file] = $project_settings_file;

[dpagini]

Oops, thanks!