feature request: localized here()

[torbjorn]

In our work we find ourselves (due to git submodules) needing to temporarily changing here() root when sourcing files.

Would a local_here( "/now/here", r_expression ) make sense? (and have r_expression being evaluated with here's root temporarily changed to said dir)

Wasn't sure to take this here (hah!) or to withr, but it sounds specific enough that it ought to go here I believe.