check for the Validator if forms are enabled

When the Form component is enabled, the container extension class
automatically enables the Validator component which therefore has to
be enabled. The container extension now throws an exception when forms
are enabled, but the Validator component is not present.
bug #11839 [FrameworkBundle] check if the Validator component is present when forms are enabled (xabbuh)

This PR was merged into the 2.5 branch.

Discussion
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[FrameworkBundle] check if the Validator component is present when forms are enabled

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

The `Symfony\Component\Validator\Validation` class is used in the
`FrameworkExtension` class. Therefore, it is required to have the
Validator component installed in production as well but not only when
being in a development environment.

Commits
-------

ed41da1 check for the Validator if forms are enabled