Cleanup HTools.Types/BasicTypes imports

Before we reorganised the source tree, the 'Result' type was exported
from HTools/Types.hs. This changed during the reorg, but at that time
we didn't change the exports; instead, we kept re-exporting it from
the old module for compatibility.

In light of future changes to the Result type, let's stop this
re-export and cleanup the imports of the other modules.

One test is slightly rewritten with explicit types declaration (part
of the values already needed one, let's make it explicit).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>