[feature request] Add extra "installer-dir" config to prepend paths destinations

[Hervé Donner]

Hello,

Composer supports overriding the vendor location using vendor-dir.
There is no equivalent for packages handled by composer/installers so everything goes to the folders specified in 'installer-paths`.

I recently made a tiny wrapper here: https://github.com/vever001/composer-installers-dir
Which, as stated, can be useful in many cases like building the project to another directory for a production release.

Would you be open to accept this feature inside composer/installers?
If so let me know and I will work on it.

Thank you

[Jordi Boggiano]

Rather not add even more complexity here sorry, for a deployment script you can run composer install and then move files where they belong IMO.