Convert pkrvars.json to pkrvars.hcl

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#### Description

Convert foo.pkrvars.json to foo.pkrvars.hcl with packer hcl2_upgrade command. Or just generally it could also be described as supporting conversion from json to hcl for any of the various json file types that packer can read now. That would probably be better.

#### Use Case(s)

I'm generating my pkrvars variable files from ansible and it's relatively easy to output json with the `to_json` filter for ansible. However, there is no such thing as a `to_hcl` filter. If I could then run packer hcl2_upgrade foo.pkrvars.json and it would generate foo.pkrvars.hcl that would help a lot. It's not really just this use case but probably dozens of sources will output json but nothing really supports hcl except packer and terraform.

The request centers on the packer hcl2_upgrade command; start by tracing its current input handling and the JSON file types it accepts. Define whether pkrvars.json is in scope or whether all supported JSON inputs should convert, then verify that the command produces valid equivalent HCL output for representative inputs.

Written by the indexing model from the issue text.