[Routing] Validate "namespace" (when using Psr4DirectoryLoader)

[Kocal]

Q	A
Branch?	7.3
Bug fix?	no
New feature?	yes
Deprecations?	no
Issues	Fix #...
License	MIT

I spend a small amount of time to understand why my controller was inaccessible:

With the following configuration:

controllers:
    resource:
        path: ../src/Application/Controller/
        namespace: App\Application/Controller
    type: attribute

You may have seen that the namespace App\Application/Controller is invalid because the presence of / instead of \. If I correct it, then my controller is accessible.

It means that invalid namespaces are simply ignored without any hints for the user, and I think it can be improved and be more user-friendly :)

This PR add namespace validation, it checks if / is found, and if namespace is composed of valid PHP identifiers:

[Kocal]

Failing checks are unrelated, they should be fixed by #59182:

[nicolas-grekas]

borrowed from YamlFileLoader in DI:

if (!preg_match('/^(?:[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+\\\\)++$/', $resource['namespace'])) {
            throw new InvalidArgumentException(\sprintf('Namespace is not a valid PSR-4 prefix: "%s".', resource['namespace']));
}

[Kocal]

Unfortunately, this modification causes the \ trimming tests to fail, whatever if I use trim($resource['namespace'], '\\') or $resource['namespace']:

I will debug a bit

[Kocal]

It works with preg_match('/^(?:[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*+\\\)++$/', trim($resource['namespace'], '\\'). '\\'), note the original \\\\ became \\\

[Kocal]

Went for a more simple solution.

Status: Needs review

Failing checks are unrelated:

[Kocal]

Status: Needs review

[fabpot]

Thank you @Kocal.