Automatically audit imports of generated clients to ensure that dependencies are explicitly declared and tested

[parthea]

https://github.com/googleapis/gapic-generator-python/issues/1922 captures the issue where google-auth is an undeclared dependency. We should audit to code to ensure that all dependencies are declared.

[vchudnov-g]

In addition, if we could automate the audit process itself somehow, we can make it less likely something like this will occur should we ever add more dependencies.

[parthea]

See related issue https://github.com/googleapis/synthtool/issues/1901 to also add this functionality downstream