fix: do not set extglob globally to avoid side-effects

Instead only enable it for whatever operation requires them.

Example sides effects:
commitpkg can accidently execute PKGBUILD functions when sourcing the
PKGBUILD that has function names like package_libsigc++()

Fixes #87

lib/common.sh

@@ -19,8 +19,6 @@ export LANG=C
 export BUILDTOOL=devtools
 export BUILDTOOLVER=m4_devtools_version
 
-shopt -s extglob
-
 # check if messages are to be printed using color
 if [[ -t 2 && "$TERM" != dumb ]]; then
 	colorize
@@ -142,12 +140,16 @@ pkgver_equal() {
 find_cached_package() {
 	local searchdirs=("$PWD" "$PKGDEST") results=()
 	local targetname=$1 targetver=$2 targetarch=$3
-	local dir pkg pkgbasename name ver rel arch r results
+	local dir pkg packages pkgbasename name ver rel arch r results
 
 	for dir in "${searchdirs[@]}"; do
 		[[ -d $dir ]] || continue
 
-		for pkg in "$dir"/*.pkg.tar?(.!(sig|*.*)); do
+		shopt -s extglob nullglob
+		mapfile -t packages < <(printf "%s\n" "$dir"/*.pkg.tar?(.!(sig|*.*)))
+		shopt -u extglob nullglob
+
+		for pkg in "${packages[@]}"; do
 			[[ -f $pkg ]] || continue
 
 			# avoid adding duplicates of the same inode

src/checkpkg.in

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-shopt -s extglob
-
 m4_include(lib/common.sh)
 
 usage() {

src/diffpkg.in

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-shopt -s extglob
-
 m4_include(lib/common.sh)
 
 usage() {

src/find-libdeps.in

@@ -5,7 +5,6 @@
 m4_include(lib/common.sh)
 
 set -e
-shopt -s extglob
 
 IGNORE_INTERNAL=0
 
@@ -43,7 +42,9 @@ fi
 
 process_sofile() {
 	# extract the library name: libfoo.so
+	shopt -s extglob nullglob
 	soname="${sofile%.so?(+(.+([0-9])))}".so
+	shopt -u extglob nullglob
 	# extract the major version: 1
 	soversion="${sofile##*\.so\.}"
 	if [[ "$soversion" = "$sofile" ]] && ((IGNORE_INTERNAL)); then