Interactive init always generates source/app.d file

[s-ludwig]

Quoting from vibe-d/vibe.d#1520:

interactive initialization generates source/app.d file. If in the middle of answering the questions, I decide to abort (i.e. ctrl-c), the source/app.d file is still there, and dub init now complains "The target directory already contains a 'source/' file. Aborting." The directory is not in a usable state.

The source directory + app.d file should not be generated until interactive initialization is complete.

Clarification: NO files should be written until interactive initialization is complete. Not just source/app.d.