repart: Allow target directory excludes

repart: Allow target directory excludes

Currently, ExcludeFiles= supports excluding directories on the host
from being copied. Let's extend this to also support preventing files
from being copied into specific directories in the partition by adding
a new option ExcludeFilesTarget=. An example where this is useful is
when setting up btrfs subvolumes in the top level that are intended to
be mounted into specific locations, so /usr would be stored in @usr,
/home in @home, .... To accomplish this, we need to copy /usr to @usr
and prevent any files from being copied into /usr in the partition,
which with this commit, we'd be able to do as follows:

```
[Partition]
CopyFiles=/usr:@usr
ExcludeFilesTarget=/usr
```