internal/pkg: move outcome dereferencing

This prepares substitute computation for shallow extern replacements.

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

internal/pkg/pkg.go

@@ -1851,6 +1851,35 @@ func (r *RContext) NewMeasuredReader(
 	return r.cache.newMeasuredReader(rc, checksum)
 }
 
+// tryChecksum dereferences a symlink to a cure outcome.
+func (c *Cache) tryChecksum(pathname *check.Absolute) (
+	checksum unique.Handle[Checksum],
+	err error,
+) {
+	_, err = os.Lstat(pathname.String())
+	if err == nil {
+		var name string
+		if name, err = os.Readlink(pathname.String()); err != nil {
+			return
+		}
+		buf := c.getIdentBuf()
+		err = Decode((*Checksum)(buf[:]), filepath.Base(name))
+		if err == nil {
+			checksum = unique.Make(Checksum(buf[:]))
+		}
+		c.putIdentBuf(buf)
+	}
+	return
+}
+
+// tryLocal attempts to obtain an [Artifact] outcome from the filesystem.
+func (c *Cache) tryLocal(id unique.Handle[ID]) (unique.Handle[Checksum], error) {
+	return c.tryChecksum(c.base.Append(
+		dirIdentifier,
+		Encode(id.Value()),
+	))
+}
+
 // tryExtern attempts to obtain an [Artifact] outcome from extern.
 func (c *Cache) tryExtern(
 	ctx context.Context,
@@ -1928,21 +1957,8 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		defer func() { c.finaliseIdent(done, id, checksum, err) }()
 	}
 
-	_, err = os.Lstat(pathname.String())
-	if err == nil {
-		var name string
-		if name, err = os.Readlink(pathname.String()); err != nil {
-			return
-		}
-		buf := c.getIdentBuf()
-		err = Decode((*Checksum)(buf[:]), filepath.Base(name))
-		if err == nil {
-			checksum = unique.Make(Checksum(buf[:]))
-		}
-		c.putIdentBuf(buf)
-		return
-	}
-	if !errors.Is(err, os.ErrNotExist) {
+	checksum, err = c.tryChecksum(pathname)
+	if err == nil || !errors.Is(err, os.ErrNotExist) {
 		return
 	}