Reorganise the lookup functions

Currently, the LookupResult, MatchPriority and related functions are
locate in Loader.hs, since (so far) only hbal needs them in the
selection of instances. However, with the new functionality on confd
side, we need these functions there too, but we don't want to import
Loader.hs (which pulls in lots of balancing-related code). So we move
all these function to BasicTypes.hs, since that module is a leaf one,
with no other dependencies.

Unittests are slightly adjusted (but they are still tested under the
'Loader' group).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>