Build configuration again extends from flake8 configuration

For backwards compatability, we need to continue to have the
build configuration extend from the flake8 configuration.
Without this, any dependencies declared as part of the flake8
configuration will be excluded from the build requirements and
thus excluded from the activatable virtual environment.

Author: elautz

pygradle-plugin/src/main/groovy/com/linkedin/gradle/python/plugin/PythonPlugin.java

@@ -155,13 +155,16 @@ private static void createConfigurations(Project project) {
 
         project.getConfigurations().create(CONFIGURATION_BOOTSTRAP_REQS.getValue());
         project.getConfigurations().create(CONFIGURATION_SETUP_REQS.getValue());
-        project.getConfigurations().create(CONFIGURATION_BUILD_REQS.getValue());
+        Configuration buildReq = project.getConfigurations().create(CONFIGURATION_BUILD_REQS.getValue());
         project.getConfigurations().create(CONFIGURATION_PYDOCS.getValue());
         project.getConfigurations().create(CONFIGURATION_TEST.getValue());
         project.getConfigurations().create(CONFIGURATION_VENV.getValue());
         project.getConfigurations().create(CONFIGURATION_WHEEL.getValue());
-        // TODO: Kept for compatibility. Remove when not needed (very soon).
-        project.getConfigurations().create("flake8");
+        // TODO: Kept for backwards compatibility. Remove when not needed (very soon).
+        // Even though flake8 is no longer in its own virtual env, users may still be adding libraries through the
+        // flake8 configuration, which still needs to be added to the build config so those libraries exist in the venv.
+        Configuration flake8 = project.getConfigurations().create("flake8");
+        buildReq.extendsFrom(flake8);
     }
 
     /*