Remove the Input flag threaded through the input graph creation process
to prevent interpolation failures on module variables.
Use an EvalOpFilter instead to inset the correct EvalNode during
walkInput. Remove the EvalTryInterpolate type, and use the same
ContinueOnErr flag as the output node for consistency and to try and
keep the number possible eval node types down.
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.
