Can no longer provide alternative implementations of ImageFetcher

[briandealwis]

Summary

PR #1216 changed ImageFetcher.Fetch() to use an internal FetchOptions type from pack/internal/image. This prevents providing alternative ImageFetcher implementationsas that pack/internal/image` package can't be imported from other modules.

This change prevents Skaffold from updating to use pack v0.20.0 as Skaffold provides an alternative implementation of ImageFetcher.

---

Reproduction

Steps

1. create fetcher.go:

package main
import "context"
import "github.com/buildpacks/pack"
import "github.com/buildpacks/pack/internal/image"
import "github.com/buildpacks/imgutil"
type fetcher struct {}
func (f *fetcher) Fetch(_ context.Context, _ string, _ image.FetchOptions) (imgutil.Image, error) { return nil, nil }
var _ pack.ImageFetcher = (*fetcher)(nil)

2. go mod init example.com/fetch
3. go mod tidy
4. go build .

Current behavior

Build fails:

$ go build
package example.com/fetcher
	fetch.go:4:8: use of internal package github.com/buildpacks/pack/internal/image not allowed

Expected behavior

Build should succeed.

[briandealwis]

This affects other types in client.go too:

• the Downloader interface's Download() returns internal/blob.Blob
• the BuildpackDownloader interface's Download() returns internal/dist.Buildpack

[aemengo]

@briandealwis Thanks for creating this issue.

How do we prefer to resolve this? In general, I'm in favor of making options public. Perhaps an ./options.go file at the root of the repository.

Any others have thoughts?

[jromero]

Tried a few things and this is going to require a bit more elbow grease (restructuring). Sorry but it may take one more release before we can ship this out. :/