Specification imports for phase0 were aliased as 'spec'. Due to the
mechanism decided upon to manage structs that vary beteween hard forks
this is no longer useful, so this removes the alias in preparation for
the Altair hard fork.
The "best" aggregate attestation strategy obtains aggregate attestations
from all listed nodes, scores them according to their attestation
coverage, and signs and broadcasts the one with the highest coverage.
The "first" aggregate attestation strategy signs and broadcasts the
first aggregate attestation returned from all listed nodes.