UnusedUse - fix false negative when namespace shortname equals imported namespace shortname

UnusedUse - fix false negative when namespace shortname equals imported namespace shortname
Looping over all namespaces and all use declarations *is* wrong. Each use declaration only cares about a single namespace declaration.

and no, in worst case scenario, running the fixer on a file with multiple namespaces in it could remove use statements in one namespace because it detected that they are from classes in the same namespace than the *other* namespace.

If you don't want to add the support for multiple namespace declarations yet in this method, the solution is to make it clear:

```php
if (empty($namespaceDeclarations)) {
    return;
}

if (count($namespaceDeclarations) > 1) {
    return; // safeguard for files with multiple namespaces to avoid breaking them until we support this case
}

$namespace = $namespaceDeclarations[0]['content'];

// The previous code goes here unchanged given that ``$namespace`` is now defined)
```

Note that if you want to keep exactly the existing behavior, the safeguard could be removed (as the existing behavior is to risk breaking files with multiple namespaces, given that it only looks at the first namespace declaration). But given that detecting the unsupported case is now easy, I would vote for including the safeguard.

Adding support for multiple namespaces in the file later will not be a matter of nesting loops to fix things, but of looking up the right namespace declaration for each use declaration before comparing the namespace