Fix askama template when called from within a macro in a different crate

[GuillaumeGomez]

Fixes #706.

This is a "two parts" problem:

1. Span::call_site gives the actual file where the macro is called, so not where the macro containing is called but where it's defined. Super weird but nothing we can do from our side.
2. To greatly improve reproducibility, we generate relative paths for include_bytes! statements we generate (so rustc rebuilds when needed) as much as possible.

So the fix here is: "if this doesn't seem to come from the same crate, then we provide the absolute path we have instead of trying to make it nicer".

If you could confirm it fixed the problem for you @cipriancraciun, it'd be super appreciated. :)

[cipriancraciun]

I can confirm that the latest main branch solves this issue.

Thanks!