internal/pkg: encode checksum in ident

This also rearranges the ident ir to be more predictable, and avoids an obvious and somewhat easy to get into inconsistent state.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -589,12 +589,21 @@ func (c *Cache) unsafeIdent(a Artifact, encodeKind bool) (
 	h := sha512.New384()
 	binary.LittleEndian.PutUint64(buf[:], uint64(a.Kind()))
 	h.Write(buf[:wordSize])
-	i := IContext{c, h}
-	a.Params(&i)
-	i.cache, i.h = nil, nil
 	for _, dn := range idents {
 		h.Write(dn[:])
 	}
+	kcBuf := c.getIdentBuf()
+	if kc, ok := a.(KnownChecksum); ok {
+		*(*Checksum)(kcBuf[:]) = kc.Checksum()
+	} else {
+		*(*Checksum)(kcBuf[:]) = Checksum{}
+	}
+	h.Write((*Checksum)(kcBuf[:])[:])
+	c.putIdentBuf(kcBuf)
+
+	i := IContext{c, h}
+	a.Params(&i)
+	i.cache, i.h = nil, nil
 
 	h.Sum(buf[wordSize:wordSize])
 	return