Fix Hackage database path for OSX and extract getHackagePath.

[aloiscochard]

Hackage database is now unified accross OS.

This fix: #2

And I would love being able to reuse the getHackagePath function in my codex library ;-)

[peti]

I've merged your code and uploaded a new release to Hackage (version 1.8).

I've add a function called hackagePath that returns the path to the Cabal home directory, as you suggested, but my function points to the 00-index.tar file rather than to the directory itself. Is that function still useful to you?

[aloiscochard]

Great @peti!

Yeah I can easily remove that last part of the path no problem :-)

Just keep in mind that others depend on that in case you do any change in future (basically please follow PVP... which you probably do already, just a friendly reminder).

Thanks again for your support!