Add --chdir flag

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#### Description

Hello! When using packer on my repo it would be very useful to be able to "chdir" o a folder when execution `packer build`.

The idea is the same on Terraform: https://developer.hashicorp.com/terraform/cli/commands#switching-working-directory-with-chdir

#### Use Case(s)

My repo structure is as follows:
```
repo/
Makefile
packer/
variables/
variables.pkrvars.hcl
frontend/
packer.pkr.hcl
backend/
packer.pkr.hcl

terraform/
terraform stuff
```

I would like to be able to execute packer from the root of my repo, from a Makefile, to make it very easy to build my images.

#### Potential configuration
An example of execution would be, using my repo structure above:
```
packer init -chdir packer/frontend/packer.pkr.hcl
packer build -chdir packer/frontend/packer.pkr.hcl -var-file packer/variables/variables.pkrvars.hcl
```

Start by tracing the CLI handling for the init and build commands, then compare the requested behavior with Terraform's chdir option. Done means packer can accept -chdir for these commands and run from the repository root while resolving the example configuration and variable paths correctly.

Written by the indexing model from the issue text.