Improve basedir calculation to fix #1274 (#1322)

* Improve basedir calculation to fix #1274

* Improve comment

Author: shadowspawn

index.js

@@ -823,7 +823,11 @@ Read more on https://git.io/JJc0W`);
     this._checkForMissingMandatoryOptions();
 
     // Want the entry script as the reference for command name and directory for searching for other files.
-    const scriptPath = this._scriptPath;
+    let scriptPath = this._scriptPath;
+    // Fallback in case not set, due to how Command created or called.
+    if (!scriptPath && process.mainModule) {
+      scriptPath = process.mainModule.filename;
+    }
 
     let baseDir;
     try {