Warn when a root constraint bypasses extra.symfony.require

[Amoifr]

Fixes the silent part of #1073.

Since #713, a root constraint that has no intersection with extra.symfony.require wins over it, on purpose: that's what allows requiring symfony/foo:dev-main while the filter is active. But the same rule kicks in when the root constraint is disjoint by mistake (e.g. "symfony/yaml": "^6.0 || ^7.4" with SYMFONY_REQUIRE=8.*): the package is silently kept unrestricted and the solver happily picks a version outside the requested range, which is how #1073 went unnoticed.

This keeps the #713 behavior untouched and simply makes the exemption loud, once per package:

Version constraint "^6.0 || ^7.4" for "symfony/yaml" has no intersection with "8.*", keeping the package unrestricted

The $this->io = null trick used to print the "Restricting packages" notice once is replaced with a boolean flag, as it would otherwise swallow warnings emitted after it.