Deprecate `exports` parameter in `gerrit_plugin_tests`

Recently, the `ext_deps` parameter was introduced, which essentially
has the same purpose as `exports`, but provides better usability. To
avoid duplication, the plugin_test_deps rule generation was adapted
to support both `ext_deps` and `exports`. `exports` should be removed
in the future.

Change-Id: Id8d54c88df63d46095ff7d13a29139dc38cb73e1

gerrit_plugin.bzl

diff --git a/gerrit_plugin.bzl b/gerrit_plugin.bzl
index 8e655b3..42a4bc9 100644
--- a/gerrit_plugin.bzl
+++ b/gerrit_plugin.bzl
@@ -246,6 +246,7 @@
       deps: List of additional Bazel dependencies for the test target.
       plugin: Name of the plugin under test. Only required if `exports`,
         `ext_deps`, or automatic plugin target wiring is used.
+        Only either `ext_deps` or `exports` is allowed.
       ext_deps: List of Maven coordinates for external test dependencies.
         When set, dependency tests are generated automatically.
       ext_repo: Name of the external repository generated by rules_jvm_external.
@@ -254,7 +255,8 @@
         is added automatically if not already present.
       exports: List of targets to export for in-tree testing. Must be used together
         with `plugin` argument. Targets will also be added as dependencies to
-        the test target created by this rule.
+        the test target created by this rule. Deprecated in favor of `ext_deps`.
+        Only either `ext_deps` or `exports` is allowed.
       skip_dependency_tests: Whether to skip generating dependency tests when
         `ext_deps` and `plugin` are set. Defaults to `False`.
       dependency_test_name: Name of the generated dependency test suite.
@@ -266,6 +268,9 @@
       **kwargs: Additional arguments passed to the underlying `junit_tests` rule.
     """
 
+    if ext_deps and exports:
+        fail("Only either provide `exports` (deprecated) or `ext_deps` (recommended).")
+
     if plugin:
         if name == None:
             name = plugin + "_tests"
@@ -280,9 +285,15 @@
     if plugin and plugin not in tags:
         tags = tags + [plugin]
 
-    if exports:
+    if plugin:
+        deps = [":%s__plugin" % plugin] + deps
+
+    if ext_deps or exports:
         if not plugin:
-            fail("plugin argument must be set when exports are provided")
+            fail("gerrit_plugin_tests: `plugin` must be set when `ext_deps` or `exports` is provided")
+        if ext_deps:
+            exports = _artifacts(ext_deps, ext_repo)
+
         java_library(
             name = plugin + "__plugin_test_deps",
             testonly = True,
@@ -291,14 +302,6 @@
         )
         deps = deps + [":" + plugin + "__plugin_test_deps"]
 
-    if plugin:
-        deps = [":%s__plugin" % plugin] + deps
-
-    if ext_deps:
-        if ext_repo == None:
-            fail("gerrit_plugin_tests: `ext_repo` must be set when `ext_deps` are provided without `plugin`")
-        deps = deps + _artifacts(ext_deps, ext_repo)
-
     junit_tests(
         name = name,
         srcs = srcs,