internal/pkg: create work directory

This is used for artifacts that cure into directories.

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

internal/pkg/pkg.go

@@ -78,6 +78,10 @@ const (
 	// dirChecksum is the directory name appended to Cache.base for storing
 	// artifacts named after their [Checksum].
 	dirChecksum = "checksum"
+
+	// dirWork is the directory name appended to Cache.base for working
+	// directories created for [Cache.Store].
+	dirWork = "work"
 )
 
 // Cache is a support layer that implementations of [Artifact] can use to store
@@ -235,6 +239,7 @@ func New(base *check.Absolute) (*Cache, error) {
 	for _, name := range []string{
 		dirIdentifier,
 		dirChecksum,
+		dirWork,
 	} {
 		if err := os.MkdirAll(base.Append(name).String(), 0700); err != nil &&
 			!errors.Is(err, os.ErrExist) {