Plugin API: Provide custom manifest file

If no manifest file is specified, Buck's java_binary() rule merges the
content of manifest files from the dependant JARs into output
META/MANIFEST.MF. Normally we wouldn't care that it ends up with a lot
of mess, but unfortunately, it breaks the plugin-api.jar, with sealed
package exception, so we do care.

This happens because we provide the same package in multiple JARs, e.g.

  com.gerrit.server.project

is shipped with plugin-api.jar, obviously, but it happens that one file
Util.class, from the same package is shipped in the

  gerrit-acceptance-framework.jar

artifact. Normally it doesn't matter, unless a JAR is defined as sealed
in which case security violation exception is thrown during unit tests
execution.

To rectify this, we use the combination of custom manifest_file
attribute of java_binary() rule and passing non documented option from
this issue: [1] to ask Buck to not merge manifest files from the
dependant JARs.

With this fix, plugin unit tests executions in standalone build mode
work again.

* [1] https://github.com/facebook/buck/issues/86

Change-Id: I7b7571c20dcf6b54210b73760eccc8e699e6f1f6
diff --git a/gerrit-plugin-api/BUCK b/gerrit-plugin-api/BUCK
index c18e497..7e12f232 100644
--- a/gerrit-plugin-api/BUCK
+++ b/gerrit-plugin-api/BUCK
@@ -13,10 +13,20 @@
 
 java_binary(
   name = 'plugin-api',
+  merge_manifests = False,
+  manifest_file = ':manifest',
   deps = [':lib'],
   visibility = ['PUBLIC'],
 )
 
+genrule(
+  name = 'manifest',
+  cmd = 'echo "Manifest-Version: 1.0" >$OUT;' +
+    'echo "Implementation-Title: Gerrit Plugin API" >>$OUT;' +
+    'echo "Implementation-Vendor: Gerrit Code Review Project" >>$OUT',
+  out = 'manifest.txt',
+)
+
 java_library(
   name = 'lib',
   exported_deps = PLUGIN_API + [