feat(gazelle)!: Move the plugin to a separate workspace (#972)

feat!(gazelle): Move the plugin to a separate workspace

Summary:
* Move go.mod to gazelle.
* Move gazelle definition.
* Fix file distribution for the gazelle module.
* Update the example test.
* Include rules_python_gazelle_plugin during integration tests
* Update ignored packages
* Update CI configuration
diff --git a/gazelle/python/python_test.go b/gazelle/python/python_test.go
index 6622bcb..e8edf89 100644
--- a/gazelle/python/python_test.go
+++ b/gazelle/python/python_test.go
@@ -38,7 +38,7 @@
 )
 
 const (
-	extensionDir      = "gazelle" + string(os.PathSeparator) + "python" + string(os.PathSeparator)
+	extensionDir      = "python" + string(os.PathSeparator)
 	testDataPath      = extensionDir + "testdata" + string(os.PathSeparator)
 	gazelleBinaryName = "gazelle_binary"
 )