Rework external protobuf dependency configuration
Rework external protobuf dependency configuration

Drop now-unused @protobuf rule from WORKSPACE. Factor out protobuf
version and checksum into variables.

googlei18n/language-resources,google/language-resources,googlei18n/language-resources,googlei18n/language-resources,google/language-resources,google/language-resources,google/language-resources,googlei18n/language-resources,google/language-resources,googlei18n/language-resources,googlei18n/language-resources,google/language-resources