Fix references to Guava classes from extension API Javadoc

ExternalIncludedIn has a reference to Multimap, so we need to add
Guava as a dependency and also provide the link to Guava's Javadocs.

Change-Id: I3aa7b7bfdb81df7b310b2ce81e7dea78e3ba0f3b

gerrit-extension-api/BUCK

@@ -1,4 +1,5 @@
 include_defs('//lib/JGIT_VERSION')
+include_defs('//lib/GUAVA_VERSION')
 
 SRC = 'src/main/java/com/google/gerrit/extensions/'
 SRCS = glob([SRC + '**/*.java'])
@@ -76,6 +77,7 @@ java_doc(
   paths = ['src/main/java'],
   srcs = SRCS,
   deps = [
+    '//lib:guava',
     '//lib/guice:javax-inject',
     '//lib/guice:guice_library',
     '//lib/guice:guice-assistedinject',
@@ -83,5 +85,5 @@ java_doc(
     '//gerrit-common:annotations',
   ],
   visibility = ['PUBLIC'],
-  external_docs = [JGIT_DOC_URL],
+  external_docs = [JGIT_DOC_URL, GUAVA_DOC_URL],
 )

lib/BUCK

@@ -1,4 +1,5 @@
 include_defs('//lib/maven.defs')
+include_defs('//lib/GUAVA_VERSION')
 
 define_license(name = 'antlr')
 define_license(name = 'Apache1.1')
@@ -66,7 +67,7 @@ maven_jar(
 
 maven_jar(
   name = 'guava',
-  id = 'com.google.guava:guava:19.0',
+  id = 'com.google.guava:guava:' + GUAVA_VERSION,
   sha1 = '6ce200f6b23222af3d8abb6b6459e6c44f4bb0e9',
   license = 'Apache2.0',
 )

lib/GUAVA_VERSION

@@ -0,0 +1,2 @@
+GUAVA_VERSION = '19.0'
+GUAVA_DOC_URL = 'https://google.github.io/guava/releases/' + GUAVA_VERSION + '/api/docs/'