fix: ensure .bzl files use relative paths (#3781)
fix: ensure .bzl files use relative paths (#3781)

I copied CreateBazelConfig.cmake from -pubsub, which is our newest
repo. This file uses relative paths for .bzl files. This is also done
by -bigquery and -spanner, so I think -cpp should likely behave
this way as well.

Part of #3560

Fixes: #3766
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp