main composer.json

{
	"name": "laravel/laravel",
	...
	"require": {
        ...
        "vapor/core": "dev-master"
	},
	...
	"minimum-stability": "dev",
        "repositories": [
            {
                "type": "git",
                "url": "[email protected]:vaporcms/core.git"
            }
        ]
}

vapor core composer.json

{
    "name": "vapor/core",
    ...
    "require": {
        ...
        "vapor/theme": "dev-master"
    },
    ...
    "repositories": [
        {
            "type": "git",
            "url": "[email protected]:vaporcms/core.git"
        },
        {
            "type": "git",
            "url": "[email protected]:vaporcms/theme.git"
        },
        {
            "type": "git",
            "url": "[email protected]:vaporcms/default-theme.git"
        }
    ]
}

vapor default-theme composer.json

{
    "name": "vapor/default-theme",
    "type": "vapor-theme",
    ...
    "require": {
        "vapor/theme": "*"
    }
}

vapor theme composer.json

{
    "name": "vapor/theme",
    "type": "composer-plugin",
    ...
    "require": {
        ...
        "vapor/core": "dev-master",
        "vapor/default-theme": "dev-master",
        "composer-plugin-api": "1.0.0"
    },
    ...
    "extra": {
        "class": "Vapor\\Theme\\Composer\\ThemeInstallerPlugin"
    },
    "repositories": [
        {
            "type": "git",
            "url": "[email protected]:vaporcms/default-theme.git"
        }
    ]
}