bug #16144 [Security] don't allow to install the split Security packages (xabbuh)
bug #16144 [Security] don't allow to install the split Security packages (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Security] don't allow to install the split Security packages

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #16134
| License       | MIT
| Doc PR        |

Currently, you would be able to install the Security component fromm
Symfony 2.3 together with one of the split packages from a higher
Symfony vesion like this:

```json
{
    "require": {
        "symfony/symfony": "2.3.*",
        "symfony/security-core": "~2.7"
    }
}
```

However, you will end up with classes being present twice.

This must be reverted after merging up in the `2.7` branch.

Commits
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0d14064 don't allow to install the split Security packages

symfony/Security