build-package: add new env. var. TERMUX_PACKAGES_DIRECTORIES

Can be set to the path to packages/ directories in other repos (like
unstable-package/packages/, termux-root-packages/packages/,
..). buildorder.py then searches these directories for packages and
dependencies

scripts/buildorder.py

@@ -236,7 +236,9 @@ def main():
     args = parser.parse_args()
     fast_build_mode = args.i
     package = args.package
-    packages_directories = args.package_dirs + ['packages']
+    packages_directories = args.package_dirs
+    if 'packages' not in packages_directories:
+        packages_directories.append('packages')
 
     if not package:
         full_buildorder = True