Improvements to our sharding support.
Improvements to our sharding support.

- Ignore tests tagged as "manual" and remove newlines before splitting.
- Correctly handle the target delimiter "--" in the config.
- Respect excluded build targets when sharding.

bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration