Handle special cases of process-distgit's target

[HauserV]

This PR proposes two minor convenience improvements to the way process-distgit handles its argument target:

1. Until 25c8f40, the command would always overwrite the original spec file. That commit introduced a new argument target, which made it possible to store the output elsewhere. This PR exposes the original behavior in the CLI, giving users the option to overwrite the spec file in-place.
2. If the target is a directory, the command copies the temporary file with the expanded spec into the directory as-is. The second improvement uses the original spec's name instead of the randomly generated temporary one.