Buck: Fix java_doc rule to use $(classpath :foo) macro

java_doc rule was using $(location :foo) instead of $(classpath :foo)
macro. This is similar but not the same. The subtle difference is the
replacement that is done for java_library() when only exported deps are
used. While location macro doesn't work here, because there are no out
file associated with such a java_library, classpath macro correctly
resolves the dependencies.

Change-Id: Iab67dff9c332027765bc0da8c46810de11974de8

tools/java_doc.defs

@@ -29,7 +29,7 @@ def java_doc(
       '-sourcepath ',
       ':'.join(sourcepath),
       ' -classpath ',
-      ':'.join(['$(location %s)' % n for n in deps]),
+      ':'.join(['$(classpath %s)' % n for n in deps]),
       '-d $TMP',
     ]) + ';jar cf $OUT -C $TMP .',
     srcs = srcs,