scripts/gen-deps: filter out grpc modules

[dfawley]

Any of grpc's modules should be fine to use anywhere, unless they pull in unwanted dependencies from outside grpc. Since the deps are generated with all transitive dependencies, simply filtering out all packages under google.golang.org/grpc should tell us what we want and not have red Xs on the presubmit statuses when nothing external changes.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.06%. Comparing base (4c07bca) to head (9b6cf7b).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7890      +/-   ##
==========================================
+ Coverage   81.84%   82.06%   +0.21%     
==========================================
  Files         377      377              
  Lines       38120    38112       -8     
==========================================
+ Hits        31201    31276      +75     
+ Misses       5603     5537      -66     
+ Partials     1316     1299      -17     

see 27 files with indirect coverage changes