internal/pkg: content-based dependency substitution

This change introduces a new fast path for FloodArtifact. It is taken when a curing artifact has identical-by-content controlled relevant inputs and are otherwise identical to an already-cured artifact.

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

internal/pkg/ir_test.go

@@ -110,6 +110,7 @@ func TestIRRoundtrip(t *testing.T) {
 				".":          {Mode: fs.ModeDir | 0700},
 				"checksum":   {Mode: fs.ModeDir | 0700},
 				"identifier": {Mode: fs.ModeDir | 0700},
+				"substitute": {Mode: fs.ModeDir | 0700},
 				"work":       {Mode: fs.ModeDir | 0700},
 			},
 		}