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/file.go

@@ -2,7 +2,6 @@ package pkg
 
 import (
 	"crypto/sha512"
-	"syscall"
 )
 
 // A fileArtifact is an [Artifact] that cures into data known ahead of time.
@@ -31,10 +30,5 @@ func (a fileArtifact) Checksum() Checksum {
 	return Checksum(h.Sum(nil))
 }
 
-// Data returns the caller-supplied data.
-func (a fileArtifact) Data() ([]byte, error) { return a, nil }
-
-// Cure returns syscall.ENOTSUP. Callers should use Data instead.
-func (a fileArtifact) Cure(*CureContext) (err error) {
-	return syscall.ENOTSUP
-}
+// Cure returns the caller-supplied data.
+func (a fileArtifact) Cure() ([]byte, error) { return a, nil }