Source directory pollution with `build: haddock: true`

[ekmett]

Building with

build: 
  haddock: true

is dropping highlight.js and style.css into my src directory.

[mgsloan]

I can indeed reproduce this, but it doesn't seem to happen for all builds. Building some dependency is causing them to get created. I suspect that this function in haddock is getting called with outdir == "." or similar. Not sure why that's happening, but the results line up.

[mgsloan]

I've created haskell/haddock#545 and worked around the issue in stack.