Hash/dependency check of multi-stage `Dockerfile` fails if unused stage references files/folders that do not exist

### Expected behavior

Unreferenced Dockerfile stages to be excluded from hash/dependency check.

### Actual behavior

Skaffold appears to do some check against the whole `Dockerfile` despite a stage not being built.

### Information

- Skaffold version: 2.2.0
- Operating system: macOS 12.6.1
- Installed via: Homebrew

### Steps to reproduce the behavior

Create a `Dockerfile` with a stage that is not built by Skaffold, pseudo code:

```
FROM ubuntu as base

FROM base as dev
# should be skipped over by build
COPY ./local_dir_that_does_not_exist ./

FROM base
# What skaffold should build without a `target` specified
```

Reference the `Dockerfile` in skaffold `build` section without specifying `target`:

```
build:
local:
push: false
useDockerCLI: true
useBuildkit: true
concurrency: 2
artifacts:
- image: ghcr.io/
context:
docker:
dockerfile: Dockerfile
```

Run `skaffold dev` and you should see output like so:

```
$ skaffold dev
Generating tags...
- ghcr.io/ -> ghcr.io/:ef6b868
Checking cache...
- ghcr.io/: Error checking cache.
Cleaning up...
Error: uninstall: Release not loaded: : release: not found
Cleaning up resources encountered an error, will continue to clean up other resources.
getting hash for artifact "ghcr.io/": getting dependencies for "ghcr.io/": file pattern [./local_dir_that_does_not_exist] must match at least one file
```