checkapk: Check how many packages depend on a previous soname

Since the changeset committed by psykose a while ago in commit
26118d4997 we now extract the
previous soname. We can pass this previous soname to `apk search -R`
to figure out how many packages are linked against this old soname.

I believe this to be useful for reviewing MRs since the checkapk
output will directly tell us if a contributor has missed rebuilds.
It will also inform us when no rebuilds are necessary, e.g. if the
SONAME was changed but no packages is linked against the library.

Example output for `main/libsodium`:

```
>>> Size difference for libsodium: 336 KiB -> 340 KiB
--- filelist-libsodium-old      2023-09-22 11:24:54.799204225 +0200
+++ filelist-libsodium-new      2023-09-22 11:24:54.799204225 +0200
@@ -1,5 +1,5 @@
 .PKGINFO
 usr/
 usr/lib/
-usr/lib/libsodium.so.23
-usr/lib/libsodium.so.23.3.0
+usr/lib/libsodium.so.26
+usr/lib/libsodium.so.26.1.0
SODIFF:
-usr/lib/libsodium.so.23.3.0:    SONAME               libsodium.so.23
+usr/lib/libsodium.so.26.1.0:    SONAME               libsodium.so.26
REBUILDS:
*** 36 packages linked against 'libsodium.so.23' need to be rebuild!
```

checkapk.in

@@ -142,8 +142,17 @@ for i in $pkgname $subpackages; do
 		# filter to things that start with -+ but strip the header (---/+++)
 		diff -U0 "filelist-$_pkgname-old" "filelist-$_pkgname-new" | grep -E '^(\+|-)[A-Za-z0-9]+' | grep '\.so' | while read -r diff_sofile; do
 			case "$diff_sofile" in
-			-*) path="$_pkgname-pkg-old"; sofile="${diff_sofile#\-}" ;;
-			+*) path="$_pkgname-pkg-new"; sofile="${diff_sofile#\+}" ;;
+			-*)
+				path="$_pkgname-pkg-old"
+				sofile="${diff_sofile#\-}"
+				soname=$(scanelf -qS "$path/$sofile" | awk '{print $1}')
+				rebuild_required=$(apk search -r --origin --exact -q "so:$soname" | sort -u | wc -l)
+				;;
+			+*)
+				path="$_pkgname-pkg-new"
+				sofile="${diff_sofile#\+}"
+				rebuild_required=0
+				;;
 			esac
 
 			# skip symlinks (only adds duplicate output or is dangling), and things that aren't valid elfs
@@ -151,8 +160,18 @@ for i in $pkgname $subpackages; do
 			if ! [ -L "$path"/"$sofile" ] && readelf -h "$path"/"$sofile" >/dev/null 2>&1; then
 				echo "$diff_sofile: " "$(objdump -p "$path"/"$sofile" | grep SONAME)"
 			fi
+
+			if [ "$rebuild_required" -ne 0 ]; then
+				echo "*** $rebuild_required packages linked against $soname need to be rebuild!" >> "$tmpdir"/rebuilds
+			fi
 		done
 	else
 		msg "No soname differences for $_pkgname."
 	fi
+
+	if [ -e "$tmpdir"/rebuilds ]; then
+		echo "REBUILDS:"
+		cat "$tmpdir"/rebuilds
+		rm "$tmpdir"/rebuilds
+	fi
 done