perf: keep only the image squash when scanning with the squashed scope

- stereoscope builds a squashed tree for every layer, each a full copy of the one below, so memory
  grows with layers x files. The squashed scope only ever reads the top one.
- new `GetSourceConfig.WithImageSquashOnly(bool)` (and `sourceproviders.Config` field) reads images
  with stereoscope's `image.WithImageSquashOnly`, scoped to that one read
- the CLI turns it on when `--scope squashed` (the default)
- an image source read this way returns an error from `FileResolver` for `all-layers` and
  `deep-squashed` instead of panicking on the missing layer squashes

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

syft/source/stereoscopesource/image_source_provider.go

@@ -15,6 +15,8 @@ type ProviderConfig struct {
 	StereoscopeImageProviderConfig stereoscope.ImageProviderConfig
 	Exclude                        source.ExcludeConfig
 	Alias                          source.Alias
+	// ImageSquashOnly reads the image keeping only the image squash, see image.WithImageSquashOnly
+	ImageSquashOnly bool
 }
 
 type stereoscopeImageSourceProvider struct {
@@ -29,6 +31,10 @@ func (l stereoscopeImageSourceProvider) Name() string {
 }
 
 func (l stereoscopeImageSourceProvider) Provide(ctx context.Context) (source.Source, error) {
+	if l.cfg.ImageSquashOnly {
+		// scoped to this read only, the caller's ctx may be shared with reads that need every layer squash
+		ctx = image.WithImageSquashOnly(ctx)
+	}
 	img, err := l.stereoscopeProvider.Provide(ctx)
 	if err != nil {
 		return nil, err