Warning for unincluded files

[siefkenj]

I just spent some time debugging my project and discovered that my docinfo.ptx was not linked to from anywhere and so it wasn't being used.

It would be nice to have a warning printed when running pretext build that prints out all the .ptx files that are not referenced by any source document. Ones that have special names, like docinfo.ptx and project.ptx could come with specific suggestions about how they are to be included.

[oscarlevin]

Maybe using a debug verbosity? My book has so many extra copies of .ptx files in various subfolders that I don't ever use, or only use for some separate build targets/versions, that I would get annoyed at the warnings.

However, I'm open to a PR that does this. It would require parsing the entire project as well as scanning the source directory.

[siefkenj]

What if it only warned for files that are included in no build targets?