The `FilesInputs` was since the beginning an XZ-specific implementation
of the input, but it was implemented in the `files_input.go` in a false
hope that it could be used as a generic stage inputs by any stages. It
turned out that various stages require different implementation of
its input. Specifically there is usually a stage-specific key, which has
assigned a common input type. For XZ stage, the key is `file`.
Remove `FilesInputs` and instead implement `XzStageInputs` which is now
accepted by the XZ stage.
Fix all affected pipeline implementations that use XZ stage.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
dd59ce6a16