This adds the new `required_plugins` block to be nested under the packer block.

Example:
```hcl
packer {
  required_plugins {
    aws = {
      version = ">= 2.7.0"
      source = "azr/aws"
    }
    azure = ">= 2.7.0"
  }
}
```

For example on darwin_amd64 Packer will install those under :
* "${PACKER_HOME_DIR}/plugin/github.com/azr/amazon/packer-plugin-amazon_2.7.0_x5.0_darwin_amd64"
* "${PACKER_HOME_DIR}/plugin/github.com/hashicorp/azure/packer-plugin-azure_2.7.0_x5.0_darwin_amd64_x5"

+ docs
+ tests

* move maps of plugins back in core

* go mod vendor

* more fixes

* fix imports

* Update core_test.go

* fix build

* more fixes

* more fixes

* up vendors after fixing sdk

* Update post_processor_mock.hcl2spec.go

* Leave implementatino of MapOf in the sdk for plugi tests

Other wise use the interface

* go mod tidy

* add MapOfDatasource type too

fix tests

Before this change loading of a packerconfig would display `loaded
plugin ...` for any plugin defined in the packerconfig - even if it
didn't exist on disk. This change updates how plugins defined in
packerconfig are loaded to ensure that only successfully loaded plugins
get the lovely message `loaded plugin ...`. For any plugin that fails to
load Packer will log that it failed to load the plugin and continue
processing any other defined plugins.

* Add a test to ensure loadSingleComponent errors when plugin does not
exists

This change introduces a loadExternalComponent which can be used for
loading a single plugin path. The function is a combination of
the discoverSingle and discoverExternalComponents functions.