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

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 1d9d5e4..b7059b6 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -34,11 +34,6 @@
     python_versions = MINOR_MAPPING.values(),
 )
 
-load("//gazelle:deps.bzl", "gazelle_deps")
-
-# gazelle:repository_macro gazelle/deps.bzl%gazelle_deps
-gazelle_deps()
-
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 # Used for Bazel CI
@@ -58,3 +53,17 @@
     name = "buildkite_config",
     toolchain = "ubuntu1804-bazel-java11",
 )
+
+local_repository(
+    name = "rules_python_gazelle_plugin",
+    path = "gazelle",
+)
+
+# The rules_python gazelle extension has some third-party go dependencies
+# which we need to fetch in order to compile it.
+load("@rules_python_gazelle_plugin//:deps.bzl", _py_gazelle_deps = "gazelle_deps")
+
+# See: https://github.com/bazelbuild/rules_python/blob/main/gazelle/README.md
+# This rule loads and compiles various go dependencies that running gazelle
+# for python requirements.
+_py_gazelle_deps()