Remove alias::getAliasOf return type as this is not compatible with PHP 7.2/7.3

src/Composer/Package/AliasPackage.php

@@ -75,7 +75,7 @@ class AliasPackage extends BasePackage
     /**
      * @return BasePackage
      */
-    public function getAliasOf(): BasePackage
+    public function getAliasOf()
     {
         return $this->aliasOf;
     }

src/Composer/Package/CompleteAliasPackage.php

@@ -35,7 +35,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
     /**
      * @return CompletePackage
      */
-    public function getAliasOf(): CompletePackage
+    public function getAliasOf()
     {
         return $this->aliasOf;
     }

src/Composer/Package/RootAliasPackage.php

@@ -35,7 +35,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @return RootPackage
      */
-    public function getAliasOf(): RootPackage
+    public function getAliasOf()
     {
         return $this->aliasOf;
     }