libmakepkg/integrity: determine what is a signature preferring local filename

Checking the file extension to determine if something is a signature is
currently done in three places:
- verify_file_signature: uses $file to print status, reuses it for
  comparison
- source_has_signatures: uses $netfile, but removes url component if
  filename component exists
- generate_one_checksum: uses $netfile and fails to detect renamed files

This leads to inconsistent behavior when trying to use a signature of
the form "foo-1.0.tar.gz.asc::https://example.com/foo-1.0.tar.gz.pgp"

Fix this by treating the third case like the second case.

Reported-by: Giancarlo Razzolini <grazzolini@archlinux.org>
Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>

scripts/libmakepkg/integrity/generate_checksum.sh.in

@@ -56,7 +56,7 @@ generate_one_checksum() {
 				sum="SKIP"
 				;;
 			*)
-				if [[ $netfile != *.@(sig?(n)|asc) ]]; then
+				if [[ ${netfile%%::*} != *.@(sig?(n)|asc) ]]; then
 					local file
 					file="$(get_filepath "$netfile")" || missing_source_file "$netfile"
 					sum="$("${integ}sum" "$file")"