Sync behaviour of extra.symfony.endpoint with SYMFONY_ENDPOINT env var

[emodric]

PR #803 says the following:

The default endpoints can be changed by setting the `extra.symfony.endpoint` entry in the root
composer.json file. The special `flex://defaults` endpoint can be used in that list to reference
the default endpoints.

On top of that, the `SYMFONY_ENDPOINT` env var can be defined to a single endpoint. This endpoint
will be used on top of (with higher priority than) the default endpoints of the app.

It would be nice if the extra.symfony.endpoint config would behave the same as the env var when using a string endpoint. Specifically, it would be nice if config like this:

"extra": {
    "symfony": {
        "endpoint": "https://api.github.com/...",
        "allow-contrib": true,
        "require": "5.3.*"
    }
}

would automatically use flex://defaults as the fallback endpoint.

This would help eliminate potential unintended disabling of the default Flex endpoint if one would configure Flex like this.

In that case, specifying the endpoint config as an array would be used to disable usage of the default endpoint if needed.

[nicolas-grekas]

Works for me, would you like to send a PR doing that?

[emodric]

Sure, I'll try to get it in today 👍