Ports: Ensure that the download filename matches the URL basename

This is a preparation step for removing support for explicitly setting a
download filename entirely.

Ports/.port_include.sh

@@ -315,6 +315,11 @@ fetch_simple() {
     filename="${2}"
     checksum="${3}"
 
+    if [ "${filename}" != "$(basename "${url}")" ]; then
+        echo "Filename '${filename}' does not match basename of '${url}'"
+        exit 1
+    fi
+
     tried_download_again=0
 
     while true; do