py-tensorflow-validation: build requires bazel, but latter not added to build dependencies

[mascguy (Christopher Nielsen)]

Failure seen when installing port from source:

:info:build running build
:info:build running bazel_build
:info:build Traceback (most recent call last):
:info:build   File "setup.py", line 148, in <module>
:info:build     setup(
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/setuptools/__init__.py", line 153, in setup
:info:build     return distutils.core.setup(**attrs)
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/core.py", line 148, in setup
:info:build     dist.run_commands()
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/dist.py", line 966, in run_commands
:info:build     self.run_command(cmd)
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/dist.py", line 985, in run_command
:info:build     cmd_obj.run()
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/command/build.py", line 135, in run
:info:build     self.run_command(cmd_name)
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/cmd.py", line 313, in run_command
:info:build     self.distribution.run_command(command)
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/dist.py", line 984, in run_command
:info:build     cmd_obj.ensure_finalized()
:info:build   File "/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/distutils/cmd.py", line 107, in ensure_finalized
:info:build     self.finalize_options()
:info:build   File "setup.py", line 70, in finalize_options
:info:build     raise RuntimeError(
:info:build RuntimeError: Could not find "bazel" binary. Please visit https://docs.bazel.build/versions/master/install.html for installation instruction.

Digging into the problem a bit: It looks like the Bazel portgroup doesn't add bazel as a build dependency, when bazel.build_cmd is set to an empty string. (And the portfile for py-tensorflow-validation sets it to empty.) The problem is, per the logs above, Bazel is needed.

The Bazel portgroup was changed on 4/13/2021, adding the aforementioned behavior:

​https://github.com/macports/macports-ports/commit/8c0bd91205487930883d7bc8c8fa866ec569964c

It's not clear whether this is a problem with the Bazel portgroup, or a combination of factors.