Fix InstalledPackageResolver cwd fallback lost by promoted property

[TomasVotruba]

InstalledPackageResolver reassigns the local $projectDirectory variable in the constructor, but the value is bound to a promoted readonly property. The reassignment never reaches $this->projectDirectory, which stays null:

public function __construct(
    private readonly ?string $projectDirectory = null
) {
    // fallback to root project directory
    if ($projectDirectory === null) {
        $projectDirectory = getcwd(); // <-- only the local variable
    }

    Assert::directory($projectDirectory);
}

resolveVendorDir() then builds null . '/vendor' = /vendor, so resolve() always throws:

The installed package json not found. Make sure you run composer update and the "vendor/composer/installed.json" file exists

This is invisible for SetManager, because RectorConfigBuilder passes getcwd() explicitly. It is hit by the autowired instance behind the freshly added ComposerPackageConstraintFilter — any rule implementing ComposerPackageConstraintInterface (#7877) currently crashes.

-    public function __construct(
-        private readonly ?string $projectDirectory = null
-    ) {
-        // fallback to root project directory
-        if ($projectDirectory === null) {
-            $projectDirectory = getcwd();
-        }
-
-        Assert::directory($projectDirectory);
-    }
+    private readonly string $projectDirectory;
+
+    public function __construct(?string $projectDirectory = null)
+    {
+        // fallback to root project directory
+        $this->projectDirectory = $projectDirectory ?? (string) getcwd();
+
+        Assert::directory($this->projectDirectory);
+    }

Covered by a new test case that constructs the resolver without arguments.