Use a separate package instead of `Path.IO` helper module

[mrkkrp]

Recently I've released path-io package that provides well-typed interface to directory and some extra functions like recursive scanning and copying of directories. I made this mainly for my personal needs, but I'll be glad if more people find it useful.

Now I've got an issue asking to add missing functions from Stack's Path.IO and proposing that Stack could switch to depending on the package. I'm fine with adding more functions to path-io if you think that depending on it is a good idea. Otherwise I just don't need those functions myself and there is no point in wasting time writing them.

In the case if you are fine with switching I could even prepare a PR for this. What do you think?

[mgsloan]

Sure! I like where you've gone with the API and docs.

If some of the functions in stack's Path.IO seem too specialized, they can get moved to Path.IO.Extra.

[mrkkrp]

Great, I'll add this to my TODO list then :-)