Fix double elaboration

Remove the requirement that FirrtlStage runs elaboration (this should
be implicit) and remove the unneeded invalidation of elaboration by
the Emitter. Due to Convert currently NOT invalidating Elaborate (when
it should), add an optionalPrerequisiteOf to ensure that the Emitter
runs before the Convert phase.

Co-authored-by: David Biancolin <david.biancolin@gmail.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>