Unbreak `cabal new-build` (#466)

Using a local `filepath` forces new-build to build the
non-local build-tool `alex` as in-place which however isn't supported properly
by `cabal new-build` yet (this will be fixed at the earliest in cabal 2.2) and thus
breaks `hadrian/build.sh` for me.

cabal.project

 packages: ./
           ../libraries/Cabal/Cabal/
-          ../libraries/filepath/
           ../libraries/text/
           ../libraries/hpc/
           ../libraries/parsec/