Keep existing constraints in package.json

[smnandre]

When a dependency constraint is already set in the package.json, the PackageSynchroniser replaces it with even if the constraint is valid.

This PR changes this behaviour and let the constraint untouched if it matches the one provided by the PHP package.

Related issues:

• https://github.com/symfony/flex/issues/843
• https://github.com/symfony/flex/issues/938

[smnandre]

Thank you both for your review and your suggestions 😃

[nicolas-grekas]

Thank you @smnandre.