Cabal library implicitly depends upon global current working directory

[bgamari]

The Cabal library (in particular configure) appears to depend implicitly upon the process's current working directory, making it rather tricky to use in a larger application. See, for instance, snowleopard/hadrian#170.

[BardurArantsson]

If I recall correctly during my work on trying to search for the .cabal file in parent directories, this is actually surprisingly non-trivial to do.

(Not that I wouldn't like to see it, but I'm just leaving this note in case anyone wants to attempt it.)

[quasicomputational]

AIUI, @angerman has solved this with #4874, and, while there are probably still lurking bugs and definitely some spots of CWD dependence, this is done well enough that Hadrian's happy with it.