internal/pkg: move dependency flooding to cache

This imposes a hard upper limit to concurrency during dependency satisfaction and moves all dependency-related code out of individual implementations of Artifact. This change also includes ctx and msg as part of Cache.

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

internal/pkg/tar_test.go

@@ -121,8 +121,8 @@ func checkTarHTTP(
 	tarDir := stubArtifact{
 		kind:   pkg.KindExec,
 		params: []byte("directory containing a single regular file"),
-		cure: func(c *pkg.CureContext) error {
-			work := c.GetWorkDir()
+		cure: func(t *pkg.TContext) error {
+			work := t.GetWorkDir()
 			if err := os.MkdirAll(work.String(), 0700); err != nil {
 				return err
 			}
@@ -136,8 +136,8 @@ func checkTarHTTP(
 	tarDirMulti := stubArtifact{
 		kind:   pkg.KindExec,
 		params: []byte("directory containing a multiple entries"),
-		cure: func(c *pkg.CureContext) error {
-			work := c.GetWorkDir()
+		cure: func(t *pkg.TContext) error {
+			work := t.GetWorkDir()
 			if err := os.MkdirAll(work.Append(
 				"garbage",
 			).String(), 0700); err != nil {
@@ -153,8 +153,8 @@ func checkTarHTTP(
 	tarDirType := stubArtifact{
 		kind:   pkg.KindExec,
 		params: []byte("directory containing a symbolic link"),
-		cure: func(c *pkg.CureContext) error {
-			work := c.GetWorkDir()
+		cure: func(t *pkg.TContext) error {
+			work := t.GetWorkDir()
 			if err := os.MkdirAll(work.String(), 0700); err != nil {
 				return err
 			}
@@ -197,7 +197,7 @@ func checkTarHTTP(
 		{"error passthrough", pkg.NewTar(stubArtifact{
 			kind:   pkg.KindExec,
 			params: []byte("doomed artifact"),
-			cure: func(c *pkg.CureContext) error {
+			cure: func(t *pkg.TContext) error {
 				return stub.UniqueError(0xcafe)
 			},
 		}, pkg.TarGzip), nil, pkg.Checksum{}, stub.UniqueError(0xcafe)},