internal/pkg: expose cure through cure context

This allows a curing Artifact to cure Artifact it depends on.

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

internal/pkg/pkg.go

@@ -78,6 +78,15 @@ func (c *CureContext) GetWorkDir() *check.Absolute { return c.work }
 // create it if they wish to use it, using [os.MkdirAll].
 func (c *CureContext) GetTempDir() *check.Absolute { return c.temp }
 
+// Cure cures the [Artifact] and returns its pathname and [Checksum].
+func (c *CureContext) Cure(a Artifact) (
+	pathname *check.Absolute,
+	checksum Checksum,
+	err error,
+) {
+	return c.cache.Cure(a)
+}
+
 // LoadData tries to load [File] from [Cache], and if that fails, obtains it via
 // [File.Data] instead. Notably, it does not cure [File].
 func (c *CureContext) LoadData(f File) (data []byte, err error) {