[constraints] Reorganize location of constraints.

Move headers from constraints/ to constraints/common/ except
interiornodeconstraints.hh.
Move constraints implementations from finiteelementmap/ to
constraints/.
Remove old (pre-PDELab 2.0) transition header gridfunctionspace/constraints.hh
[Constraints] Drop "constraints" in file name of constraints implementations.

The headers are already located in the folder constraints/.