internal/pkg: exclusive artifacts

This alleviates scheduler overhead when curing many artifacts.

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

internal/pkg/tar.go

@@ -80,6 +80,9 @@ func (a *tarArtifact) Dependencies() []Artifact {
 	return []Artifact{a.f}
 }
 
+// IsExclusive returns false: decompressor and tar reader are fully sequential.
+func (a *tarArtifact) IsExclusive() bool { return false }
+
 // A DisallowedTypeflagError describes a disallowed typeflag encountered while
 // unpacking a tarball.
 type DisallowedTypeflagError byte