Bug triggered by simple name of package

[66Ton99]

If I create custom pacage from git for exaple. And give name like "name": "sfPhpunitPlugin" it throw nitice in Composer\Installers\BaseInstaller

If I give name like "name": "plugin/sfPhpunitPlugin" it works fine but you should display specific message or add support for simple name.

[shama]

I assumed all Composer packages used vendor/name. I'll write a fix for this. Thanks for reporting!