Migrate DrupalAutoloader off deprecated drupal-finder v1 APIs

[mglaman]

Problem

src/Drupal/DrupalAutoloader.php uses DrupalFinder\DrupalFinder and its locateRoot() method, which were deprecated in drupal-finder 1.3.0 and removed in 2.0.0. Currently phpstan-baseline.neon suppresses 5 errors related to this usage:

• Call to deprecated method locateRoot() of class DrupalFinder\DrupalFinder
• Instantiation of deprecated class DrupalFinder\DrupalFinder
• Two type mismatch errors caused by the old API returning bool|string

When the composer constraint eventually allows drupal-finder 2.x, this will cause a hard runtime crash during analysis.

Proposed resolution

Replace DrupalFinder\DrupalFinder with DrupalFinder\DrupalFinderComposerRuntime, which accepts a starting path in its constructor and provides the same getDrupalRoot() / getVendorDir() methods without the deprecated locateRoot() call.

// Before
$finder = new DrupalFinder();
$finder->locateRoot(getcwd());
$this->drupalRoot = $finder->getDrupalRoot();

// After
$finder = new DrupalFinderComposerRuntime();
$this->drupalRoot = $finder->getDrupalRoot();

After migrating, the 5 baseline suppressions in phpstan-baseline.neon for DrupalAutoloader.php should be removed.