Closes #320: Extensions in sites/<site-dir>/modules not discovered and autoloaded

[dpagini]

ExtensionDiscovery doesn't scan multisite directories, causing false "module is not found" errors

Problem

ExtensionDiscovery hardcodes the site path to sites/default:

$this->sitePath = 'sites/default';

In a Drupal multisite setup, modules can live under other site-specific directories like sites/acme/modules/. Because ExtensionDiscovery::scan() only searches sites/default for the ORIGIN_SITE weight, modules in any other site directory are never added to the ExtensionMap.

This causes the LoadIncludes rule (and ModuleLoadInclude) to report false positives:

File acme_module.inc could not be loaded from Drupal\Core\Extension\ModuleHandlerInterface::loadInclude because acme_module module is not found.

Steps to reproduce

1. Place a module under a non-default site directory, e.g. sites/acme/modules/acme_module/
2. Reference that module via loadInclude():

   \Drupal::moduleHandler()->loadInclude('acme_module', 'inc');

3. Run PHPStan — the module is not found

Failing test

❌ Failing test evidence for commit 718ef91

Branch 320 adds a test demonstrating this bug. The test places a fixture module under tests/fixtures/drupal/sites/acme/modules/ and asserts that loadInclude() resolves it without errors. It currently fails:

FAILURES!
Tests: 6, Assertions: 6, Failures: 1

1) LoadIncludesRuleTest::test with data set "multisite module in sites/acme/"
  File acme_module.inc could not be loaded from
  Drupal\Core\Extension\ModuleHandlerInterface::loadInclude
  because acme_module module is not found.

Proposed fix

Replace the single $sitePath property with discovery of all site directories under sites/*/. In scan(), iterate through each discovered site path with the ORIGIN_SITE weight, skipping sites/all (already handled) and sites/simpletest (irrelevant).

This mirrors what Drupal core's own ExtensionDiscovery does when no specific site path is provided — it scans all site directories.

Disclaimer: some contents written with help of AI

[dpagini]

@aweingarten just pinging you here b/c you opened #320 (even though it was 6 years ago). Not sure if you have any thoughts on this one...