Relative Path Support for spotlessFiles

[t-rad679]

As discussed extensively from this comment in issue 529 onward, it would be nice to add some kind of support for specifying relative paths in spotlessFiles, which currently only works with absolute paths. The documentation should be very clear about how this will function in all cases, including what directory the path should be relative to.

After some discussion in the other ticket, I think it makes the most sense for the path to be relative to the root directory of the root project.

[nedtwigg]

Whatever discussion or work happens in service of this ticket, please make sure that it is driven by a usecase that you have. E.g. you are connecting to a tool and it is difficult to generate the absolute paths which are currently neeeded. If you have a usecase, and can solve your problem without creating new problems for the people who are already using the -PspotlessFiles feature, then you can merge whatever.

[nedtwigg]

With the work happening in #568, I'm more convinced than ever that relative path support is likely to cause problems. I think this issue is mostly driven by frustration with the surprising regex behavior, which is a separate problem.

[t-rad679]

Ok, sounds good! Thanks, Ned!

[nedtwigg]

FYI, -PspotlessFiles has been deprecated and will be removed. Migration path available here: #602