Remove unsafePerformIO usage

We need to change a few things, most importantly CLI options defaults,
but otherwise we already used the path to files from functions which
were already in the I/O monad, so we don't have to change much of the
code flow.

Additionally, Path.hs now has an explicit export list, to stop
accidental leakage of symbols.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>