internal/pkg: exclusive artifacts

This alleviates scheduler overhead when curing many artifacts.

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

internal/pkg/pkg_test.go

@@ -96,12 +96,14 @@ func (a *stubArtifact) Kind() pkg.Kind               { return a.kind }
 func (a *stubArtifact) Params(ctx *pkg.IContext)     { ctx.GetHash().Write(a.params) }
 func (a *stubArtifact) Dependencies() []pkg.Artifact { return a.deps }
 func (a *stubArtifact) Cure(t *pkg.TContext) error   { return a.cure(t) }
+func (*stubArtifact) IsExclusive() bool              { return false }
 
 // A stubArtifactF implements [FloodArtifact] with hardcoded behaviour.
 type stubArtifactF struct {
 	kind   pkg.Kind
 	params []byte
 	deps   []pkg.Artifact
+	excl   bool
 
 	cure func(f *pkg.FContext) error
 }
@@ -110,6 +112,7 @@ func (a *stubArtifactF) Kind() pkg.Kind               { return a.kind }
 func (a *stubArtifactF) Params(ctx *pkg.IContext)     { ctx.GetHash().Write(a.params) }
 func (a *stubArtifactF) Dependencies() []pkg.Artifact { return a.deps }
 func (a *stubArtifactF) Cure(f *pkg.FContext) error   { return a.cure(f) }
+func (a *stubArtifactF) IsExclusive() bool            { return a.excl }
 
 // A stubFile implements [FileArtifact] with hardcoded behaviour.
 type stubFile struct {