"Your package name My Repository is invalid": bug or breaking change?

Hello,

Using the satis.json from the docs at https://getcomposer.org/doc/articles/handling-private-packages.md#setup :

```json
{
"name": "My Repository",
"homepage": "http://packages.example.org",
"repositories": [
{ "type": "vcs", "url": "https://github.com/mycompany/privaterepo" },
{ "type": "vcs", "url": "http://svn.example.org/private/repo" },
{ "type": "vcs", "url": "https://github.com/mycompany/privaterepo2" }
],
"require-all": true
}
```

And the latest container tag:

```
Digest: sha256:0f6cb23de613edf42bfe4da17333b5ada6bb99e21c20324ed91db5f13bb9e744
Status: Downloaded newer image for composer/satis:latest
```

I get this error:

```
docker run --rm --init -it \
--user $(id -u):$(id -g) \
--volume $(pwd):/build \
composer/satis build satis.json foo

In RootPackageLoader.php line 76:

Your package name My Repository is invalid, it should have a vendor name, a forward slash, and a package name. The vendor and package name can be words separated by -, . or _. The complete name shoul
d match "^[a-z0-9]([_.-]?[a-z0-9]+)*/[a-z0-9](([_.]?|-{0,2})[a-z0-9]+)*$".

build [--repository-url [REPOSITORY-URL]] [--repository-strict] [--no-html-output] [--skip-errors] [--stats] [--] [ [ [...]]]

```

Is this a bug or a breaking change?

Cheers!