Alter health/reflection packages' build criterion

Using the presence of the `*_pb2_grpc` module, as opposed to the absence
of the build script (`*_commands` module) is a problematic choice,
because even if a generated file is present, the test infrastructure may
want to regenerate it under a different environment (e.g. different
Python/proto package version).  This will ensure the protos always get
recompiled if we have a `*_commands` module present, signaling we are in
a build environment, thereby making the process hermetic.

src/python/grpcio_health_checking/setup.py

@@ -60,17 +60,8 @@ INSTALL_REQUIRES = ('protobuf>=3.3.0',
                     'grpcio>={version}'.format(version=grpc_version.VERSION),)
 
 try:
-    # ensure we can load the _pb2_grpc module:
-    from grpc_health.v1 import health_pb2_grpc as _pb2_grpc
-    # if we can find the _pb2_grpc module, the package has already been built.
-    SETUP_REQUIRES = ()
-    COMMAND_CLASS = {
-        # wire up commands to no-op not to break the external dependencies
-        'preprocess': _NoOpCommand,
-        'build_package_protos': _NoOpCommand,
-    }
-except ImportError:  # we are in the build environment
     import health_commands as _health_commands
+    # we are in the build environment, otherwise the above import fails
     SETUP_REQUIRES = (
         'grpcio-tools=={version}'.format(version=grpc_version.VERSION),)
     COMMAND_CLASS = {
@@ -78,6 +69,13 @@ except ImportError:  # we are in the build environment
         'preprocess': _health_commands.CopyProtoModules,
         'build_package_protos': _health_commands.BuildPackageProtos,
     }
+except ImportError:
+    SETUP_REQUIRES = ()
+    COMMAND_CLASS = {
+        # wire up commands to no-op not to break the external dependencies
+        'preprocess': _NoOpCommand,
+        'build_package_protos': _NoOpCommand,
+    }
 
 setuptools.setup(
     name='grpcio-health-checking',

src/python/grpcio_reflection/setup.py

@@ -61,17 +61,8 @@ INSTALL_REQUIRES = ('protobuf>=3.3.0',
                     'grpcio>={version}'.format(version=grpc_version.VERSION),)
 
 try:
-    # ensure we can load the _pb2_grpc module:
-    from grpc_reflection.v1alpha import reflection_pb2_grpc as _pb2_grpc
-    # if we can find the _pb2_grpc module, the package has already been built.
-    SETUP_REQUIRES = ()
-    COMMAND_CLASS = {
-        # wire up commands to no-op not to break the external dependencies
-        'preprocess': _NoOpCommand,
-        'build_package_protos': _NoOpCommand,
-    }
-except ImportError:  # we are in the build environment
     import reflection_commands as _reflection_commands
+    # we are in the build environment, otherwise the above import fails
     SETUP_REQUIRES = (
         'grpcio-tools=={version}'.format(version=grpc_version.VERSION),)
     COMMAND_CLASS = {
@@ -79,6 +70,13 @@ except ImportError:  # we are in the build environment
         'preprocess': _reflection_commands.CopyProtoModules,
         'build_package_protos': _reflection_commands.BuildPackageProtos,
     }
+except ImportError:
+    SETUP_REQUIRES = ()
+    COMMAND_CLASS = {
+        # wire up commands to no-op not to break the external dependencies
+        'preprocess': _NoOpCommand,
+        'build_package_protos': _NoOpCommand,
+    }
 
 setuptools.setup(
     name='grpcio-reflection',