Delete modules in case of breaking changes or invalid checksums, compatibility with go mod download (#10)

* add module invalidation via DELETE request
* add go mod download vcs provider

pkg/store/store.go

@@ -14,6 +14,7 @@ type logger = func(...interface{})
 type Store interface {
 	Put(ctx context.Context, snapshot Snapshot) error
 	Get(ctx context.Context, module string, version vcs.Version) (Snapshot, error)
+	Del(ctx context.Context, module string, version vcs.Version) error
 	Close() error
 }