This commit should have no effect on behavior.  It splits the OpenGoal type into
OpenGoal and PotentialGoal.  Both types are only used when building the search
tree.  While PotentialGoal can represent any dependency or constraint, OpenGoal
only represents dependencies that introduce goals (solver variables).  Limiting
OpenGoal to only represent goals simplifies the code.

This commit also removes OpenGoal and FlaggedDep's 'comp' type variables.  The
two types had only been used with 'comp' equal to () or Component.  Now
FlaggedDep always uses Component, and OpenGoal does not need a Component.