File provisioner with lxd builder acts if trailing slash is present

_This issue was originally opened by @markododa as hashicorp/packer#7530. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

Using the lxd builder, i have a simple file provisioner decleration

`{
"type": "file",
"source": "/root/saltstack/reactor",
"destination": "/srv/reactor"
},
`
as per the documentation at https://www.packer.io/docs/provisioners/file.html this should create a folder /srv/reactor with the contents of the source directory.
However /srv/reactor is not created but its contents are in the /srv folder