This follows #8232 which added the code to generate the code required to parse
HCL files for each packer component.

All old config files of packer will keep on working the same. Packer takes one
argument. When a directory is passed, all files in the folder with a name
ending with  “.pkr.hcl” or “.pkr.json” will be parsed using the HCL2 format.
When a file ending with “.pkr.hcl” or “.pkr.json” is passed it will be parsed
using the HCL2 format. For every other case; the old packer style will be used.

## 1. the hcl2template pkg can create a packer.Build from a set of HCL (v2) files

I had to make the packer.coreBuild (which is our one and only packer.Build ) a public struct with public fields

## 2. Components interfaces get a new ConfigSpec Method to read a file from an HCL file.

  This is a breaking change for packer plugins.

a packer component can be a: builder/provisioner/post-processor

each component interface now gets a `ConfigSpec() hcldec.ObjectSpec`
which allows packer to tell what is the layout of the hcl2 config meant
to configure that specific component.

This ObjectSpec is sent through the wire (RPC) and a cty.Value is now
sent through the already existing configuration entrypoints:

 Provisioner.Prepare(raws ...interface{}) error
 Builder.Prepare(raws ...interface{}) ([]string, error)
 PostProcessor.Configure(raws ...interface{}) error

close #1768


Example hcl files:

```hcl
// file amazon-ebs-kms-key/run.pkr.hcl
build {
    sources = [
        "source.amazon-ebs.first",
    ]

    provisioner "shell" {
        inline = [
            "sleep 5"
        ]
    }

    post-processor "shell-local" {
        inline = [
            "sleep 5"
        ]
    }
}

// amazon-ebs-kms-key/source.pkr.hcl

source "amazon-ebs" "first" {

    ami_name = "hcl2-test"
    region = "us-east-1"
    instance_type = "t2.micro"

    kms_key_id = "c729958f-c6ba-44cd-ab39-35ab68ce0a6c"
    encrypt_boot = true
    source_ami_filter {
        filters {
          virtualization-type = "hvm"
          name =  "amzn-ami-hvm-????.??.?.????????-x86_64-gp2"
          root-device-type = "ebs"
        }
        most_recent = true
        owners = ["amazon"]
    }
    launch_block_device_mappings {
        device_name = "/dev/xvda"
        volume_size = 20
        volume_type = "gp2"
        delete_on_termination = "true"
    }
    launch_block_device_mappings {
        device_name = "/dev/xvdf"
        volume_size = 500
        volume_type = "gp2"
        delete_on_termination = true
        encrypted = true
    }

    ami_regions = ["eu-central-1"]
    run_tags {
        Name = "packer-solr-something"
        stack-name = "DevOps Tools"
    }
    
    communicator = "ssh"
    ssh_pty = true
    ssh_username = "ec2-user"
    associate_public_ip_address = true
}
```