internal/pkg: optionally exempt implementations from cures counter

This avoids holding up many slots with a long pipeline.

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

internal/pkg/pkg.go

@@ -485,6 +485,16 @@ type KnownChecksum interface {
 	Checksum() Checksum
 }
 
+// CuresExempt is optionally implemented for an artifact exempt to the
+// cache-wide cures counter and limit.
+type CuresExempt interface {
+	Artifact
+
+	// CuresExempt is a no-op function but serves to distinguish implementations
+	// that are cures-exempt.
+	CuresExempt()
+}
+
 // FileArtifact refers to an [Artifact] backed by a single file.
 //
 // FileArtifact does not support fine-grained cancellation. Its context is
@@ -1892,6 +1902,10 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		}
 	}()
 
+	if _, ok := a.(CuresExempt); ok {
+		curesExempt = true
+	}
+
 	var (
 		ctx  context.Context
 		done chan<- struct{}