Rework output related logic

Rework output related logic

- Let's store the output as a "base" name without any suffixes from
which we construct all other output paths.
- Let's not allow --output to be specified as a path anymore and
always put it in the configured output directory.
- Let's return all output paths as strings instead of paths. As
strings, we'll get typing errors if we try to use these as paths
without joining them with a directory first, which can be either the
output directory or the staging directory.
- Let's always create a symlink from the "base" name to the full
output path so it can be referred to regardless of the output
format, compression or image version that's used.
- Let's make sure we compress split partitions if requested