add pkr plugin check cmd from packer core

[azr]

This is simplified to not pull in core as a dep. The drawback is that we don't actually start plugins to verify that it works with Packer and has a ConfigSpec func. We do start a binary though.

But I think this is fine since if a plugin implements describe then we can be pretty sure it uses the SDK's function and interfaces.

Related to hashicorp/packer#11317

closes hashicorp/packer#11309

[nywilken]

This looks great. I like the approach of checking the describe output. In the future we might be able to extended it a bit to catch any potential name issues. For example some plugin not using plugin.DEFAULT_NAME for a single component.

Nice work.

[nywilken]

When testing against a built plugin not in my GoPath I get an error.

~>  tree 
├── packer-plugin-azure
├── packer-plugin-upcloud_v1.2.0_x5.0_darwin_amd64
└── packer-sdc

~>  ./packer-sdc plugin-check packer-plugin-upcloud_v1.2.0_x5.0_darwin_amd64
2021/10/08 12:09:50 failed to describe plugin: exec: "packer-plugin-upcloud_v1.2.0_x5.0_darwin_amd64": executable file not found in $PATH

When we execute describe we might need to determine the absolute path of the binary.

[azr]

When testing against a built plugin not in my GoPath I get an error.

Good catch, that's weird, I wonder why. It's fixed using the absolute path.