Buck: Fix eclipse project generation

After refactoring done in Ia46c0559a lib/jgit is recognized as source
folder for the non existing Edit patch library:

  $ buck audit classpath //gerrit-gwtui:ui_module
  buck-out/gen/lib/jgit/lib__Edit__output/Edit.jar

As the consequence non existing path is induced from it and included
in .classpath file:

  path="/home/davido/projects/gerrit/lib/jgit/src/main/java"

Add a check for existence of source folder and include it only if the
folder exists.

Change-Id: Icef39196173722c9c8b760c041a4c9ec9d22ab7c

tools/eclipse/project.py

@@ -178,7 +178,8 @@ def gen_classpath():
         classpathentry('lib', j, s)
   for s in sorted(gwt_src):
     p = path.join(ROOT, s, 'src', 'main', 'java')
-    classpathentry('lib', p, out='buck-out/eclipse/gwtsrc')
+    if path.exists(p):
+      classpathentry('lib', p, out='buck-out/eclipse/gwtsrc')
 
   classpathentry('con', JRE)
   classpathentry('output', 'buck-out/eclipse/classes')