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
f3285e0280