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() hcld...