Add option for extensions to be composer-only

Background

Some extensions may use external packages installed via composer and don't want to include those libraries into the extension as phar or as complete directories.

Acceptance criterias

• Option to mark extension as "composer only"
• Remove option to download as ZIP
• Replace download button in list with alternative
• Marked extensions are not listed at the Extension Manager
• Ensure that author can only add own (or allowed - how?) packages
• Ensure uniqueness of package name (deny package, if we have an extension record with same package name)
• Use packagist API to fetch package details