This add :
* discovery of `packer-plugin-*` binaries from the known folders and ask them to describe themselves
* tests
For testing: in go we create a bash script that in turn calls back to Go. I could not make the tests to work on windows and then would like to postpone testing this for when we know more about the finite layout of this feature. That is mainly: how things are going to work with init, versioning and such.