Copy fails when we try to copy to the root of the destination registry

**What steps did you take:**
When copying an image to a registry which has no repository or no folder beneath it, it fails.
`imgpkg copy -i nginx --to-repo ip-10-0-0-146.ap-south-1.compute.internal/` or
`imgpkg copy -i nginx --to-repo ip-10-0-0-146.ap-south-1.compute.internal`

**What happened:**
The image copy fails with the following error message:
```
root@ip-172-32-0-87:~# imgpkg copy -i nginx --to-repo ip-10-0-0-146.ap-south-1.compute.internal/
0 B / ? [---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------=] 0.00% 2562047h47m16s
copy | done importing images
Error: Building import repository ref: repository must be between 2 and 255 runes in length:
```

```
root@ip-172-32-0-87:~# imgpkg copy -i nginx --to-repo ip-10-0-0-146.ap-south-1.compute.internal
copy | exporting 1 images...
copy | will export index.docker.io/library/nginx@sha256:1c70a669bbf07f9862f269162d776c35144b116938d1becb4e4676270cff8f75
copy | exported 1 images
copy | importing 1 images...
panic: send on closed channel
panic: close of closed channel

goroutine 1 [running]:
github.com/google/go-containerregistry/pkg/v1/remote.sendError(...)
github.com/google/go-containerregistry@v0.5.0/pkg/v1/remote/write.go:185
github.com/google/go-containerregistry/pkg/v1/remote.MultiWrite.func1(0xc000274380, 0xc0000e1360)
github.com/google/go-containerregistry@v0.5.0/pkg/v1/remote/multi_write.go:99 +0x85
github.com/google/go-containerregistry/pkg/v1/remote.MultiWrite(0xc00008e270, 0xc0002367c0, 0x4, 0x4, 0x98e400, 0xc0003030c0)
github.com/google/go-containerregistry@v0.5.0/pkg/v1/remote/multi_write.go:160 +0x1220
github.com/k14s/imgpkg/pkg/imgpkg/registry.Registry.MultiWrite.func1(0x3b9aca00, 0xc00009e1c0)
github.com/k14s/imgpkg/pkg/imgpkg/registry/registry.go:112 +0x89
github.com/k14s/imgpkg/pkg/imgpkg/util.Retry(0xc0000e1438, 0x60, 0xc025e0)
github.com/k14s/imgpkg/pkg/imgpkg/util/retry.go:25 +0xa3
github.com/k14s/imgpkg/pkg/imgpkg/registry.Registry.MultiWrite(0xc0002367c0, 0x3, 0x4, 0x0, 0x0, 0x0, 0xc00008e270, 0x5, 0x44132e, 0xc0000e14e0)
github.com/k14s/imgpkg/pkg/imgpkg/registry/registry.go:111 +0x99
github.com/k14s/imgpkg/pkg/imgpkg/imageset.(*ImageSet).Import(0xc0004c53e0, 0xc000030540, 0x1, 0x1, 0x0, 0x90079e, 0xf, 0x7ffdb9e077ee, 0x29, 0x99a458, ...)
github.com/k14s/imgpkg/pkg/imgpkg/imageset/image_set.go:114 +0x342
github.com/k14s/imgpkg/pkg/imgpkg/imageset.ImageSet.Relocate(0x5, 0x98e580, 0xc00026c1e0, 0xc00000e0a8, 0x0, 0x90079e, 0xf, 0x7ffdb9e077ee, 0x29, 0x99a458, ...)
github.com/k14s/imgpkg/pkg/imgpkg/imageset/image_set.go:48 +0x1d6
github.com/k14s/imgpkg/pkg/imgpkg/cmd.CopyRepoSrc.CopyToRepo(0x7ffdb9e077de, 0x5, 0x0, 0x0, 0x0, 0x0, 0x0, 0x5, 0x992360, 0xc00026c1e0, ...)
github.com/k14s/imgpkg/pkg/imgpkg/cmd/copy_repo_src.go:57 +0x257
github.com/k14s/imgpkg/pkg/imgpkg/cmd.(*CopyOptions).Run(0xc000150000, 0x0, 0x0)
github.com/k14s/imgpkg/pkg/imgpkg/cmd/copy.go:140 +0xd28
github.com/k14s/imgpkg/pkg/imgpkg/cmd.NewCopyCmd.func1(0xc000142f00, 0xc00009f080, 0x0, 0x4, 0x0, 0x0)
github.com/k14s/imgpkg/pkg/imgpkg/cmd/copy.go:44 +0x2a
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc000142f00, 0xc00009f080, 0x0, 0x4, 0x0, 0x0)
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0xb2
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc000142f00, 0xc00009f080, 0x0, 0x4, 0x0, 0x0)
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0xb2
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc000142f00, 0xc00009f080, 0x0, 0x4, 0x0, 0x0)
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0xb2
github.com/spf13/cobra.(*Command).execute(0xc000142f00, 0xc00009f040, 0x4, 0x4, 0xc000142f00, 0xc00009f040)
github.com/spf13/cobra@v1.1.3/command.go:852 +0x472
github.com/spf13/cobra.(*Command).ExecuteC(0xc000142500, 0xc000142500, 0xc00009ecc0, 0xc000040748)
github.com/spf13/cobra@v1.1.3/command.go:960 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
github.com/spf13/cobra@v1.1.3/command.go:897
main.main()
github.com/k14s/imgpkg/cmd/imgpkg/imgpkg.go:30 +0x188
```

**What did you expect:**
It should succeed or display a clear error message stating the reason for failure.

**Anything else you would like to add:**
NA

**Environment:**

- imgpkg version (use `imgpkg --version`): imgpkg version 0.8.0
- Docker registry used (e.g. `Docker HUB`): source is Docker Hub and destination registry is private docker registry
- OS (e.g. from `/etc/os-release`): Ubuntu 20.04.2 LTS (Focal Fossa)

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