File provisioner incorrectly embeds source/ directory into dest when downloading

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#### Overview of the Issue

File provisioner appears to embed source directory within destination when source directotory is specified with a trailing slash, and *direction* is *download*, contrary to the docs.

The issue appears to only be present when `direction = "download"`. When uploading, it works as documented.

#### Reproduction Steps

```hcl
provisioner "file" {
direction = "download"
source = "~/source/"
destination = "./dest"
}
```
results in downloading files under `./dest/source/`, instead of under `./dest/` as specified in [documentation](https://developer.hashicorp.com/packer/docs/provisioners/file#directory-uploads).

Result is the same whether or not destination includes a trailing slash.

### Packer version

`Packer v1.8.3` (slightly out of date, but I couldn't find any related bug report so haven't tried updating).

### Operating system and Environment details

Both source and destination systems run Ubuntu 22.04.

Start with Packer's file provisioner download path and reproduce the configuration from the issue on Ubuntu. Compare the result with the linked directory-upload documentation; the work is done when a source directory with a trailing slash downloads its contents directly under the destination without adding the source directory name.

Written by the indexing model from the issue text.