Fetching @gapic_generator_python_pip_deps takes 5+ minutes

[aohren]

Breaking this out as a separate issue from #361 (where I first mentioned this).

When trying to build the py_gapic_library Bazel rule, the Fetching @gapic_generator_python_pip_deps step takes a little >5 mins from a clean build every time.

Reproducible by doing bazel clean --expunge before building.

[hkdevandla]

I guess @vam-google is looking into this. Vadym, please update with your findings.