build: register mock dependency with Bazel

[wchargin]

Summary:
We’ve heretofore used the mock library without telling Bazel about it.
This isn’t generally a problem when tensorflow is installed in the
same virtualenv, because tensorflow pulls in mock. But tests that
don’t depend on TensorFlow have no build edge to mock when imported
into google3’s build system, causing build failures after syncing. This
commit makes the mock dependency explict.

We use mock==1.0.0 rather than the current version (2.0.0) because the
current version depends on pbr at runtime in a way that does not play
well with Bazel: testing-cabal/mock#383
(It appears to be trying to find some globally registered setup.cfg
file from which to read a version identifier.) Rather than vendoring and
patching mock, declaring a fake :expect_mock_installed target, or
hacking PBR_VERSION=2.0.0 into the action env, we simply depend on a
saner version of the library, which is also the version used in google3.

Existing mock users have been detected and fixed up:

$ bazel query "
>     kind(
>         py_.*rule,
>         rdeps(//..., set($(git grep -l 'import mock\($\| \)')), 1)
>     )
> " \
> | awk '{ print "add deps @org_pythonhosted_mock|" $0 }' \
> | buildozer -f - \
> ;

Test Plan:
All notf tests now pass in a virtualenv that does not include mock:

$ virtualenv -q -p python2 ./ve
$ . ./ve/bin/activate
(ve) $ pip install 'absl-py>=0.7.0' 'numpy<2.0,>=1.14.5' boto3==1.9.86 flake8==3.5.0 futures==3.1.1 grpcio==1.6.3 yamllint==1.5.0 >/dev/null 2>&1
(ve) $ python -c 'import mock'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/buildtools/current/sitecustomize/sitecustomize.py", line 152, in SetupPathsAndImport
    return real_import(name, globals, locals, fromlist, level)
ImportError: No module named mock
(ve) $ cd ~/git/tensorboard
(ve) $ bazel test //tensorboard/... --test_tag_filters=support_notf
...
INFO: Build completed successfully, 12 total actions
//tensorboard:lazy_test                                                  PASSED in 0.2s
//tensorboard:lib_test                                                   PASSED in 0.9s
//tensorboard:manager_test                                               PASSED in 0.3s
//tensorboard:plugin_util_test                                           PASSED in 0.5s
//tensorboard:program_test                                               PASSED in 0.8s
//tensorboard/backend:application_test                                   PASSED in 1.0s
//tensorboard/backend:http_util_test                                     PASSED in 0.8s
//tensorboard/backend:json_util_test                                     PASSED in 0.8s
//tensorboard/compat/tensorflow_stub:gfile_test                          PASSED in 0.8s
//tensorboard/summary:summary_test                                       PASSED in 0.8s
//tensorboard/util:platform_util_test                                    PASSED in 0.3s

Previously, manager_test and application_test failed in such an
environment.

wchargin-branch: mock-dep

[wchargin]

Googlers: http://cl/244441864 is the corresponding Copybara change.

[nfelt]

Thank you!! 🥝

[wchargin]

cc @orionr, in case you add more support_notf tests and run into this.
No action required.