Introduce Composer merge tool for local dependencies. (#1425)

composer.json

@@ -31,6 +31,7 @@
         "vufind-org/vufinddate": "1.0.0",
         "vufind-org/vufindharvest": "2.4.1",
         "vufind-org/vufindhttp": "2.2.0",
+        "wikimedia/composer-merge-plugin": "1.4.1",
         "yajra/laravel-pdo-via-oci8": "1.4.0",
         "zendframework/zend-cache": "2.8.2",
         "zendframework/zend-captcha": "2.9.0",

composer.local.json.dist

+{
+    "__comment__": [
+        "Rename this file from composer.local.json.dist to composer.local.json to"
+        "manage your local dependencies without changing the core composer.json."
+        "See https://github.com/wikimedia/composer-merge-plugin for more details."
+    ],
+    "require": {
+        "example/dependency": "1.0.0"
+    }
+}