Support for Packer plugin dependencies

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

As of Packer 1.7.0, users can [explicitly list Packer plugin dependencies in the `packer` block of the Packer template file](https://developer.hashicorp.com/packer/docs/templates/hcl_templates/blocks/packer#specifying-plugin-requirements). (Eventually [this will be required](https://developer.hashicorp.com/packer/guides/1.7-template-upgrade#what-if-you-only-use-components-that-are-built-into-the-packer-core).) It would be great if Dependabot could track these versions and create its usual helpful pull requests as new versions are released.

Start by locating the Packer template handling for the `packer` block and existing dependency-manager support. Confirm how plugin dependency versions are read, then verify that supported updates produce the usual Dependabot pull requests for newer plugin versions.

Written by the indexing model from the issue text.