Package should require `composer/installers`

[zoliszabo]

From Composer's documentation (https://getcomposer.org/doc/faqs/how-do-i-install-a-package-to-a-custom-path-for-my-framework.md#how-do-i-install-a-package-to-a-custom-path-for-my-framework-):

If you are a package author and want your package installed to a custom directory, simply require composer/installers and set the appropriate type.