Allow plugins to contribute external workspace deps

When building a Gerrit plugin in-tree, allow to add extra dependencies
to the WORKSPACE file.

The only operation required will be the overwrite of the
plugins/external_plugin_deps.bzl file with the one provided by the
plugin.

Bug: Issue 5305
Change-Id: I07e4538f9f81ef923a4ca1aeceaee75d8429434e

Documentation/dev-build-plugins.txt

@@ -56,9 +56,10 @@ Repeat step 1. above.
 
 
 The fact that plugin contains `BUILD` file doesn't mean that building this
-plugin from the plugin directory works. For now it doesn't. Bazel in tree driven
-means it can only be built from within Gerrit tree. Clone or link the plugin
-into gerrit/plugins directory:
+plugin from the plugin directory works.
+
+Bazel in tree driven means it can only be built from within Gerrit tree. Clone
+or link the plugin into gerrit/plugins directory:
 
 ----
 cd gerrit
@@ -74,6 +75,26 @@ bazel-genfiles/plugins/<plugin-name>/<plugin-name>.jar
 Some plugins describe their build process in `src/main/resources/Documentation/build.md`
 file. It may worth checking.
 
+=== Plugins with external dependencies ===
+
+If the plugin has external dependencies, then they must be included from Gerrit's
+own WORKSPACE file. This can be achieved by including them in `external_plugin_deps.bzl`.
+During the build in Gerrit tree, this file must be copied over the dummy one in
+`plugins` directory.
+
+Example for content of `external_plugin_deps.bzl` file:
+
+----
+load("//tools/bzl:maven_jar.bzl", "maven_jar")
+
+def external_plugin_deps():
+  maven_jar(
+      name = 'org_apache_tika_tika_core',
+      artifact = 'org.apache.tika:tika-core:1.12',
+      sha1 = '5ab95580d22fe1dee79cffbcd98bb509a32da09b',
+  )
+----
+
 == Bazel standalone driven
 
 Only few plugins support that mode for now:

WORKSPACE

@@ -2,6 +2,7 @@ workspace(name = "gerrit")
 
 load("//tools/bzl:maven_jar.bzl", "maven_jar", "GERRIT", "MAVEN_LOCAL")
 load("//lib/codemirror:cm.bzl", "CM_VERSION", "DIFF_MATCH_PATCH_VERSION")
+load("//plugins:external_plugin_deps.bzl", "external_plugin_deps")
 
 ANTLR_VERS = "3.5.2"
 
@@ -1104,3 +1105,4 @@ bower_archive(
 load("//lib/js:bower_archives.bzl", "load_bower_archives")
 
 load_bower_archives()
+external_plugin_deps()

plugins/external_plugin_deps.bzl

@@ -0,0 +1,2 @@
+def external_plugin_deps():
+    pass