extract_domains does the wrong thing with interpolate.

Consider the following Firedrake MFE:

```python
from firedrake import *
from firedrake.__future__ import interpolate

mesh = UnitSquareMesh(2, 2)
Q = FunctionSpace(mesh, "CG", 1)
u = Function(Q)
vom = VertexOnlyMesh(mesh, ([0.5, 0.5],))
P0DG = FunctionSpace(vom, "DG", 0)

domain = interpolate(u, P0DG).ufl_domain()
```

Here `domain` is `Mesh(VectorElement(FiniteElement('Lagrange', triangle, 1), dim=2), 1)` but it should be `Mesh(VectorElement(FiniteElement('Discontinuous Lagrange', Cell(vertex, 2), 0), dim=2), 8)`. I.e. the domain of `u` is being returned but it should be the domain of `P0DG`. This is because `extract_domains` assumes that domains come from terminals, which is not true in this case. A similar issue will arise for external operator.