Add the following snippet to ~/.composer/config.json:

{
  "repositories": [
        {
            "type": "path",
            "url": "/path/to/package",
            "options": {
                "versions": {
                    "owner/repo": "3.1.10"
                },
                "symlink": false
            }
        }
    ]
}

versions is optional and can be used to replace a specific package version when required. symlink can be set to true or false depending on what's required when developing.

References:

• https://schibsted.com/blog/using-local-packages-as-composer-dependencies
• https://getcomposer.org/doc/05-repositories.md#path