Unwrap sourcerefs in a temporary directory with original file layout

[lionel-]

The temp directory would be kept around so it's only unwrapped once per session. Then ESS and RStudio can use this to jump to source for all packages with dangling srcrefs (e.g. packages installed from CRAN when R_KEEP_PKG_SOURCE is set).

Might be worth writing in pure base R and proposed for inclusion in the utils package.