Remove uses of get_distribution to better support package vendoring

[tswast]

get_distribution doesn't function when packages are vendored. We should remove uses of this function.

python-bigquery/google/cloud/bigquery/__init__.py

Line 33 in d1eb8b3

__version__ = get_distribution("google-cloud-bigquery").version

See: googleapis/python-api-core#80 as an example of how to fix this using a "version" module. Release tooling is being updated to support this method.