The search path of Nix contains by default the "nix" prefix that points
to $datadir/nix/corepkgs, where $datadir defaults to $prefix/lib at
installation time, but can be overriden by NIX_DATA_DIR.
We implemented it using `Paths.hnix.getDataDir` and `NIX_DATA_DIR` to
follow Nix behaviour as closely as possible.
A small discrepancy is that we do the lookup on each invocation, where
Nix caches the searchPath at context creation.
Note that this is an enforced public API change.
I went with changing the `MonadFile` typeclass rather than threading
through a `MonadFail` at every use-site, but that would be possible as
well if it makes more semantic sense - however, since `MonadFile`
already seems to require an IO-ish implementation, adding that
superclass doesn't seem like an imposition.
