Do not include the same stub multiple times

[kocsismate]

This is a prerequisite for #5195

I haven't done all the changes for intl. But if the current approach is suitable, then I'll continue :)

[nikic]

Looks basically fine. I'm not entirely happy about the intl part, I'm wondering if we should move all the functions into php_intl.stub.php, as that's where they are registered.

(Generally, I'm not a fan of the over-separation going on in the intl implementation, always opening multiple files until I find where something is defined.)

[kocsismate]

To be honest, I like it very much that the function and method counterparts are close to each other now. Although, the fact that they won't reside in the same file anymore after this PR makes my argument less compelling... So I'm ok to move function stubs to php_intl.stub.php.

[kocsismate]

As a next step, I'll annotate the alias functions.