fix(bazel): do not reference repo name

[dbolduc]

Dependents may give our repo any name. We should not be referencing our own project's name.

Bug introduced as part of #14173

To make the changes, I ran:

sed -i "s;@google_cloud_cpp//bazel:gapic.bzl;//bazel:gapic.bzl;" $(git ls-files)

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.79%. Comparing base (146cf3c) to head (b2b1c49).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14287      +/-   ##
==========================================
+ Coverage   93.24%   93.79%   +0.55%     
==========================================
  Files        2214     2301      +87     
  Lines      193630   204402   +10772     
==========================================
+ Hits       180550   191727   +11177     
+ Misses      13080    12675     -405     

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[scotthart]

Reviewed 127 of 127 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbolduc)