Revert "Avoid conflicting java_library rules with multiple gerrit_plugin_tests()"

The java_library rule was actually required for classpath creation.

This reverts commit 7d6806975ed4ea9ecdf04adbb20041f802619519.

Change-Id: I4bfcf2da520f1b07b58ad7eb1c347a8d938c2f40

gerrit_plugin.bzl

diff --git a/gerrit_plugin.bzl b/gerrit_plugin.bzl
index 9abb526..c59ce2b 100644
--- a/gerrit_plugin.bzl
+++ b/gerrit_plugin.bzl
@@ -290,13 +290,19 @@
     if plugin:
         deps = [":%s__plugin" % plugin] + deps
 
-    if ext_deps:
-        if not ext_repo:
-            fail("gerrit_plugin_tests: `plugin` or `ext_repo` must be set when `ext_deps` is provided")
-        deps = deps + _artifacts(ext_deps, ext_repo)
+    if ext_deps or exports:
+        if not plugin:
+            fail("gerrit_plugin_tests: `plugin` must be set when `ext_deps` or `exports` is provided")
+        if ext_deps:
+            exports = _artifacts(ext_deps, ext_repo)
 
-    if exports:
-        deps = deps + exports
+        java_library(
+            name = plugin + "__plugin_test_deps",
+            testonly = True,
+            visibility = ["//visibility:public"],
+            exports = exports,
+        )
+        deps = deps + [":" + plugin + "__plugin_test_deps"]
 
     junit_tests(
         name = name,