Improve class_alias robustness

[guilliamxavier]

• Use ::class rather than string literal, to avoid problems like Remove space in class_alias #313
• Use distinct alias names (concatenate current file class + external class) rather than class_exists, to avoid conflicts without triggering autoload looking for nonexistent files like "src/EventListener/UserContextRequestEvent.php"

[guilliamxavier]

I will try to make PHPStan happy (after meal)...

[guilliamxavier]

After searching and trying many things, there appears to be a limitation of PHPStan with ::class on undefined classes (even with class_exists / class_alias), e.g. the following snippet:

<?php
namespace My\Ns;
class Foo {}
if (!class_exists(Bar::class)) {
    class_alias(Foo::class, Bar::class);
}
function f(Bar $x): void {
    var_dump($x);
}
f(new Foo());

is valid and will output (https://3v4l.org/pPO3n)

object(My\Ns\Foo)#1 (0) {
}

but PHPStan complains (https://phpstan.org/r/b7443e0a-20b9-4e77-8710-51538dfbea57)

+--------------------------------------------------------------------------------+
| Line | test.php                                                                |
+--------------------------------------------------------------------------------+
| 4    | Class My\Ns\Bar not found.                                              |
| 5    | Class My\Ns\Bar not found.                                              |
| 7    | Parameter $x of function My\Ns\f() has invalid typehint type My\Ns\Bar. |
| 10   | Parameter #1 $x of function My\Ns\f expects My\Ns\Bar, My\Ns\Foo given. |
+--------------------------------------------------------------------------------+

 [ERROR] Found 4 errors 

So I ended up ignoring the errors of https://travis-ci.org/getsentry/sentry-symfony/jobs/643821562#L644