cmd/cp: Fix --from-oci-layout when using full reference

Copying an image from an OCI layout with a value like
"foo/bar:latest" for "org.opencontainers.image.ref.name" was failing with:

```
$ oras cp --from-oci-layout ./myimage:foo/bar:latest localhost:5000/foo:latest
Error: invalid argument "./myimage:foo/bar:latest": failed to find path "./myimage:foo/bar:": stat /myimage:foo/bar: no such file or directory
```

According to [0], foo/bar:latest if a valid value for
"org.opencontainers.image.ref.name", so the command should work.

This commit fixes that by using strings.Index() instead of strings.LastIndex().

[0]: https://github.com/opencontainers/image-spec/blob/v1.1.0-rc5/annotations.md

Signed-off-by: Mauricio Vásquez <mauriciov@microsoft.com>

cmd/oras/internal/option/target.go

@@ -38,7 +38,6 @@ import (
 	"oras.land/oras-go/v2/registry/remote/auth"
 	"oras.land/oras-go/v2/registry/remote/errcode"
 	oerrors "oras.land/oras/cmd/oras/internal/errors"
-	"oras.land/oras/cmd/oras/internal/fileref"
 )
 
 const (
@@ -123,20 +122,22 @@ func (opts *Target) Parse(cmd *cobra.Command) error {
 // parseOCILayoutReference parses the raw in format of <path>[:<tag>|@<digest>]
 func (opts *Target) parseOCILayoutReference() error {
 	raw := opts.RawReference
-	var path string
+	path := raw
 	var ref string
 	if idx := strings.LastIndex(raw, "@"); idx != -1 {
 		// `digest` found
 		path = raw[:idx]
 		ref = raw[idx+1:]
-	} else {
-		// find `tag`
-		var err error
-		path, ref, err = fileref.Parse(raw, "")
-		if err != nil {
-			return errors.Join(err, errdef.ErrInvalidReference)
-		}
+	} else if idx := strings.Index(raw, ":"); idx != -1 {
+		// `tag` found
+		path = raw[:idx]
+		ref = raw[idx+1:]
 	}
+
+	if path == "" {
+		return fmt.Errorf("found empty file path in %q", raw)
+	}
+
 	opts.Path = path
 	opts.Reference = ref
 	return nil

cmd/oras/internal/option/target_test.go

@@ -78,11 +78,12 @@ func Test_parseOCILayoutReference(t *testing.T) {
 	}{
 		{"Empty input", "", "", "", true},
 		{"Empty path and tag", ":", "", "", true},
-		{"Empty path and digest", "@", "", "", false},
+		{"Empty path and digest", "@", "", "", true},
 		{"Empty digest", "path@", "path", "", false},
 		{"Empty tag", "path:", "path", "", false},
 		{"path and digest", "path@digest", "path", "digest", false},
 		{"path and tag", "path:tag", "path", "tag", false},
+		{"path with full reference", "path:repo:tag", "path", "repo:tag", false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {