restore composer xmlprovider installer

Author: ioigoume

composer.json

@@ -45,7 +45,8 @@
         "simplesamlphp/assert": "~1.9.0"
     },
     "require-dev": {
-        "simplesamlphp/simplesamlphp-test-framework": "~1.10.1"
+        "simplesamlphp/simplesamlphp-test-framework": "~1.10.1",
+        "simplesamlphp/composer-xmlprovider-installer": "~1.1.0"
     },
     "support": {
         "issues": "https://github.com/simplesamlphp/xml-common/issues",
@@ -55,7 +56,8 @@
         "allow-plugins": {
             "composer/package-versions-deprecated": true,
             "dealerdirect/phpcodesniffer-composer-installer": true,
-            "phpstan/extension-installer": true
+            "phpstan/extension-installer": true,
+            "simplesamlphp/composer-xmlprovider-installer": true
         }
     },
     "scripts": {
@@ -64,7 +66,7 @@
             "vendor/bin/composer-require-checker check --config-file=tools/composer-require-checker.json composer.json",
             "vendor/bin/phpstan analyze -c phpstan.neon",
             "vendor/bin/phpstan analyze -c phpstan-dev.neon",
-            "vendor/bin/composer-unused",
+            "vendor/bin/composer-unused --excludePackage=simplesamlphp/composer-xmlprovider-installer",
             "vendor/bin/phpunit --no-coverage --testdox"
         ],
         "tests": [