Plugin target-dir error handling for source-file element

In #576 & #577 there is an open question about what if a character other than `/` comes after a `target-dir` prefix such as `app`, `libs`, or `src/main` in a `source-file` element.

I think we should consider aborting with an error in this case _and in case of some other invalid `target-dir` prefixes_.

I wonder if we should consider a similar form of error handling on the other platforms.

Start by reviewing issues #576 and #577 and the existing source-file handling for target-dir prefixes such as app, libs, and src/main. Determine the intended behavior for invalid prefixes or following characters, and whether comparable handling is needed on other platforms; done means the behavior and error cases are defined and covered.

Written by the indexing model from the issue text.