Warn if filepath is outside the crate

[Morganamilo]

We have a workspace that contains .mdd files and a bunch of crates. The crates reference these files via mermaid!() which works but as soon as we try to publish these crates it then errors because the crate is uploaded individually and can't access files outside the crate.

I would be nice if this can be detected and warned about/error before hand instead of this bubbling up on publish.

I've been trying to think of how this could be done and I don't think it can be without converting this to a proc macro which would add a ton of complexity. I've opened the issue anyway incase there's any ideas and to mention the problem.

[glueball]

I'll investigate. Maybe could find/create some replacement for include_str and "proc-macro" only that part, since that problem might be more general than mermaid.