wakebox: visible paths should not be absolute

[mmjconolly]

If a visible files list is provided to wakebox via a spec json containing an absolute path, wakebox itself should error-out.
Currently appears to silently ignore them.

[mmjconolly]

and also any other files that are outside the workspace, using ../ within the path names of visible files, though I suspect that's never been a problem outside of hand-crafted wakebox spec files

[mmjconolly]

It's possible that people have written build rules that have absolute Paths that point outside the workspace, since they were previously ignored this would be a break in the API

[V-FEXrt]

It's possible that people have written build rules that have absolute Paths that point outside the workspace, since they were previously ignored this would be a break in the API

There are several steps to get there, but I'd like to eventually make it impossible to construct a path outside of the workspace. That should at least help in this case