Bugfix: avoid deleting possibly current working directory
package subcommands: pick the package name from current or specified directory
Executable not found errors now report executable name
Refactoring: exceptions
generate-package-spec: do not require any option or parameter by default
Keep generated spec files in src/ so that output/ can be ignored by git.

Side-effect: output/ can be used in OBS directly now.
Bump to 0.24.0