Report cache filename at download

In certain cases, a user wants to know the cached filename to add the file themselves,
see https://github.com/pyenv/pyenv/issues/1743 .
Since we report both a filename and a URL anyway, there's no reason to report a wrong one.

plugins/python-build/bin/python-build

@@ -417,8 +417,8 @@ fetch_tarball() {
    fi
 
   if ! reuse_existing_tarball "$package_filename" "$checksum"; then
-    local tarball_filename="$(basename "$package_url")"
-    echo "Downloading ${tarball_filename}..." >&2
+    # Report the cached file name -- sometimes, it's useful to know (#1743)
+    echo "Downloading ${package_filename}..." >&2
     http head "$mirror_url" &&
     download_tarball "$mirror_url" "$package_filename" "$checksum" ||
     download_tarball "$package_url" "$package_filename" "$checksum"