Allow module variables to fail interpolation during input. This is OK
since they will be verified again during Plan. Because Input happens
before Refresh, module variable interpolation can fail when referencing
values that aren't yet in the state, but are expected after Refresh.
Variables weren't being interpolated during the Input phase, causing a
syntax error on the interpolation string. Adding `walkInput` to the
EvalTree operations prevents skipping the interpolation step.
