xbps-src: implement a by_sha256 hash cache

common/xbps-src/shutils/update_hash_cache.sh

@@ -0,0 +1,12 @@
+# vim: set ts=4 sw=4 et:
+
+update_hash_cache() {
+    local cache="$XBPS_SRCDISTDIR/by_sha256"
+    local distfile curfile
+    mkdir -p "$cache"
+    find "$XBPS_SRCDISTDIR" -type f | grep -v by_sha256 | while read -r distfile; do
+        cksum=$(cat "$distfile" | sha256sum | cut -d " " -f 1)
+        curfile="$(basename ${distfile})"
+        ln -vf "$distfile" "${cache}/${cksum}_${curfile}"
+    done
+}