Clarify build docs and error messages, remove unnecessary file name restrictions

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#### Description

In short, the problems are:
1. The Packer Build docs do not explain how `-var-file` works or link to any other documentation on supported arguments
2. Error messages from Packer do not explain how to format `.hcl` formatted `-var-file`
3. Packer is requiring specific file names for variables files, rather than detecting them from the file itself, making it impossible to keep configuration DRY (re-using the same format configuration file from Terraform)

Here is an example of the problem:

```
$ packer build -var-file terraform.sh.tfvars ../../../../modules/root/aws-jenkins-cluster/provisioning/ami.pkr.hcl
Error: Could not guess format of terraform.sh.tfvars

A var file must be suffixed with `.hcl` or `.json`.
```

It appears that `-var-file` in Packer is different than `-var-file` in Terraform, and this is frustrating.

In Terraform, a `-var-file` can either be a `.tfvars`-style configuration file, or a `.tfvars.json`-style configuration file. I do not believe the Terraform files require a specific file extension to detect the correct file type.

According to the [Packer build docs](https://www.packer.io/docs/commands/build#var-file), the `-var-file` option is to *"Set template variables from a file"*. It does not describe what kind of files this option takes, but according to the above error, it is either an `hcl` file or a `json` file.

Using an `hcl` file here doesn't appear to make sense. HCL is a template language, but `-var-file` is to set template variables. Does this mean the `-var-file` option is supposed to be passed an HCL file which only contains `variable "name" {}` blocks? No idea based on the Build docs.

More searching finally reveals [this section on HCL template variables](https://www.packer.io/docs/templates/hcl_templates/variables#variable-definitions-pkrvars-hcl-and-auto-pkrvars-hcl-files). Apparently you can use a `.tfvars`-style variables file here. But according to the above error, only if the file name is formatted a specific way. This prevents it from re-using an existing Terraform file, even though it's the same file format. The alternative, using JSON for both with a `.json` extension, is problematic because editing JSON files by hand leads to unexpected syntax errors, whereas a `.tfvars`-style key-value file is easier to edit without introducing syntax errors.

Composeable command-line tools should not force users into unnecessary restrictions on their use of the tools. It creates UX headaches, adds use-case complexity, and limits the usability of the tools.

#### Use Case(s)

Sharing one configuration values file for Packer and Terraform.

#### Potential configuration

##### ami.pkr.hcl
```hcl
source "amazon-ebs" "tio_base_amazonlinux2" {
ami_name = "foo"
instance_type = "t2.micro"
region = var.aws_region
subnet_filter {
filters = {
"tag:Name": var.private_subnet_name
}
}
}
```

##### terraform.tfvars
```hcl
aws_region = "us-east-1"
vpc_id = "vpc-xxxxxxxxxx"
availability_zone = "us-east-1a"
private_subnet_name = "myvpc.subnet.private"
```

##### terraform.tf
```hcl
data "aws_subnet" "private" {
vpc_id = var.vpc_id
availability_zone = var.availability_zone
filter {
name = "tag:Name"
values = [var.private_subnet_name]
}
}
```

#### Potential References