perf: build lower layer squashes only when a scan reads them

The image source provider now reads images with `image.WithLazyLayerSquashes`: the image squash is built during the read, and each lower layer's squash only when something reads it. A squashed scan never does, so it skips a full file tree copy per layer.

- squashed scans: faster, much less memory on images with many layers
- `scan`/`attest` with all-layers or deep-squashed stay eager (an internal ctx flag), since eager squashing overlaps with fetching and indexing and is faster when every layer gets read
- library callers going through `syft.GetSource` get lazy for every scope, since the scope isn't known when the image is read. All-layers is still correct, just slower
- no public API change

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

cmd/syft/internal/commands/scan.go

@@ -260,6 +260,11 @@ func getSource(ctx context.Context, opts *options.Catalog, userInput string, sou
 		cfg = cfg.WithDigestAlgorithms(hashers...)
 	}
 
+	if source.ParseScope(opts.Scope) != source.SquashedScope {
+		// every other scope reads each layer's squash, which is cheaper to build while the image is read
+		ctx = internal.WithEagerLayerSquashes(ctx)
+	}
+
 	src, err := syft.GetSource(ctx, userInput, cfg)
 	if err != nil {
 		return nil, fmt.Errorf("could not determine source: %w", err)