Excluding the .imgpkg folder of a bundle does not allow pushing as an image

**What steps did you take:**
Create a bundle directory (bundle_dir) including the necessary .imgpkg dir and images.yml file
Attempt to push the bundle directory as an image
`imgpkg push -i -f bundle_dir`
Observe the expected error.
> ...consider using a bundle

Now, exclude the .imgpkg directory from the push command.
`imgpkg push -i -f bundle_dir --file-exclude-defaults .imgpkg`

**What happened:**
The same error.
> ...consider using a bundle

**What did you expect:**
The directory and its contents pushed as an image to the repo.

Unsure if this is a pathway we want to allow/encourage, it's a little strange to be pushing a bundle as an image, but I would expect the tool to respect my "exclude" choices.

**Anything else you would like to add:**
This happens because of where our excluded files are checked, only when making the tarball, after the image/bundle verification steps.

**Environment:**

- imgpkg version (use `imgpkg --version`): imgpkg version 0.2.0
- Docker registry used (e.g. `Docker HUB`): Any repo
- OS (e.g. from `/etc/os-release`): Mac