Split Cradle Logic into smaller components

[fendor]

API Changes:

-- | Given root/foo/bar.hs, return root/hie.yaml, or wherever the yaml file was found
findCradle :: FilePath -> IO (Maybe FilePath)

-- | Given root/hie.yaml load the Cradle
loadCradle :: FilePath -> IO Cradle

-- | Given root/foo/bar.hs, load an implicit cradle
loadImplicitCradle :: FilePath -> IO Cradle

Currently assumes, #26 will be merged, but can be rebased if necessary.
closes #9 afaik.
cc @ndmitchell