You might want to have a dependency installed locally when doing composer installs. This might help for example for library development.
You can do so by adding a repositories section to composer.
Assuming the package you want have local is called namespace/package and you'll install it in repository/package, you can modify the composer.json file as follows:
{
	"repositories": [
    	{
      		"type": "path",
      		"url":  "repository/package",
      		"options": {
        		"symlink": true
      		}
    	}
    ]
}
Composer will then look into your local repository for package.
You might want to:
composer clearcache
composer remove namespace/package
composer install namespace/package
To have the package removed from the vendor dir and symlinked locally.