duffle export should no longer store images in tar files

Thus the --oci-layout flag is no longer needed.

Also improve the wording of the help text.

Ref cnabio#767

cmd/duffle/export.go

@@ -4,8 +4,6 @@ import (
 	"fmt"
 	"io"
 
-	"github.com/pkg/errors"
-
 	"github.com/deislabs/duffle/pkg/duffle/home"
 	"github.com/deislabs/duffle/pkg/loader"
 	"github.com/deislabs/duffle/pkg/packager"
@@ -14,28 +12,19 @@ import (
 )
 
 const exportDesc = `
-Packages a bundle, and by default any images referenced by the bundle, within
-a single gzipped tarfile.
-
-If neither --oci-layout nor --thin is specified, all images (incuding invocation
-images) referenced by the bundle metadata are saved as tar files in the
-artifacts/ directory along with an artifacts.json file which describes the
-contents of the artifacts/ directory.
+Packages a bundle, and by default any images referenced by the bundle, within a single gzipped tarfile.
 
-If --oci-layout is specified, all images (incuding invocation images) referenced
-by the bundle metadata are saved as an OCI image layout in the artifacts/layout/
-directory.
+Unless --thin is specified, a thick bundle is exported. A thick bundle contains the bundle manifest and all images
+(including invocation images) referenced by the bundle metadata. Images are saved as an OCI image layout in the
+artifacts/layout/ directory.
 
 If --thin specified, only the bundle manifest is exported.
 
-By default, this command will use the name and version information of
-the bundle to create a compressed archive file called
-<name>-<version>.tgz in the current directory. This destination can be
-updated by specifying a file path to save the compressed bundle to using
-the --output-file flag.
+By default, this command will use the name and version information of the bundle to create a compressed archive file
+called <name>-<version>.tgz in the current directory. This destination can be updated by specifying a file path to save
+the compressed bundle to using the --output-file flag.
 
-Pass in a path to a bundle file instead of a bundle in local storage by
-using the --bundle-is-file flag like below:
+A path to a bundle file may be passed in instead of a bundle in local storage by using the --bundle-is-file flag, thus:
 $ duffle export [PATH] --bundle-is-file
 `
 
@@ -47,7 +36,6 @@ type exportCmd struct {
 	thin         bool
 	verbose      bool
 	bundleIsFile bool
-	ociLayout    bool
 }
 
 func newExportCmd(w io.Writer) *cobra.Command {
@@ -61,18 +49,14 @@ func newExportCmd(w io.Writer) *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			export.home = home.Home(homePath())
 			export.bundle = args[0]
-			if export.thin && export.ociLayout {
-				return errors.New("--thin and --oci-layout must not both be specified")
-			}
 
 			return export.run()
 		},
 	}
 
 	f := cmd.Flags()
 	f.StringVarP(&export.dest, "output-file", "o", "", "Save exported bundle to file path")
-	f.BoolVarP(&export.bundleIsFile, "bundle-is-file", "f", false, "Indicates that the bundle source is a file path")
-	f.BoolVarP(&export.ociLayout, "oci-layout", "l", false, "Export images as an OCI image layout")
+	f.BoolVarP(&export.bundleIsFile, "bundle-is-file", "f", false, "Interpret the bundle source as a file path")
 	f.BoolVarP(&export.thin, "thin", "t", false, "Export only the bundle manifest")
 	f.BoolVarP(&export.verbose, "verbose", "v", false, "Verbose output")
 
@@ -92,7 +76,7 @@ func (ex *exportCmd) run() error {
 }
 
 func (ex *exportCmd) Export(bundlefile string, l loader.BundleLoader) error {
-	is, err := packager.NewImageStore(ex.thin, ex.ociLayout)
+	is, err := packager.NewImageStore(ex.thin, true)
 	if err != nil {
 		return err
 	}