Add plugin module paths to system path

Closes tianocore#338

Adds dynamically loaded plugin module directory paths to the system
path so the plugin modules can import other modules relative to their
path as they would if they were not dynamically loaded.

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

edk2toolext/environment/plugin_manager.py

@@ -95,6 +95,11 @@ def _load(self, PluginDescriptor):
                 py_module_name, py_module_path)
             module = importlib.util.module_from_spec(spec)
             sys.modules[py_module_name] = module
+
+            py_module_dir = os.path.dirname(py_module_path)
+            if py_module_dir not in sys.path:
+                sys.path.append(py_module_dir)
+
             spec.loader.exec_module(module)
         except Exception:
             exc_info = sys.exc_info()