Changed name of python module: ufl -> ufl_legacy
------------------------------------------------

The upstream python module for UFL is simply called ufl, and is
provided in Debian by the python3-ufl package.

At version 2023.1.0, ufl (supporting next-generation ffcx and dolfinx)
became incompatible with legacy ffc and dolfin.

For this reason the legacy version ufl 2022.2.0 has been packaged
separately as python3-ufl-legacy in order to continue supporting ffc
and dolfin.

The python module name has been changed from ufl to ufl_legacy.

The python3-dolfin and python3-ffc packages have been patched to match
this change. The dolfin module should continue to run without
requiring changes to user scripts.

However if your own scripts use the legacy ufl python module directly,
you will need to update them to use ufl_legacy instead.

All ufl import statements will need to be updated to ufl_legacy.

That is,
  import ufl
can be changed to
  import ufl_legacy as ufl
requiring no further changes in the use of the ufl object.

Any explicit imports must likewise be updated
e.g. change
  from ufl import product
to
  from ufl_legacy import product
