internal/pkg: expose response body

This uses the new measured reader provided by Cache. This should make httpArtifact zero-copy.

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

internal/pkg/pkg_test.go

@@ -119,7 +119,7 @@ type stubFile struct {
 	stubArtifact
 }
 
-func (a *stubFile) Cure(context.Context) (io.ReadCloser, error) {
+func (a *stubFile) Cure(*pkg.RContext) (io.ReadCloser, error) {
 	return io.NopCloser(bytes.NewReader(a.data)), a.err
 }