classpath.bzl: Avoid using deprecated depset union

Change-Id: I0ad36f82a2e6324e303205bccf8eaee2f3969952

tools/bzl/classpath.bzl

diff --git a/tools/bzl/classpath.bzl b/tools/bzl/classpath.bzl
index 9ec5f15..55479c5 100644
--- a/tools/bzl/classpath.bzl
+++ b/tools/bzl/classpath.bzl
@@ -1,13 +1,13 @@
 def _classpath_collector(ctx):
-    all = depset()
+    all = []
     for d in ctx.attr.deps:
         if hasattr(d, "java"):
-            all += d.java.transitive_runtime_deps
-            all += d.java.compilation_info.runtime_classpath
+            all.append(d.java.transitive_runtime_deps)
+            all.append(d.java.compilation_info.runtime_classpath)
         elif hasattr(d, "files"):
-            all += d.files
+            all.append(d.files)
 
-    as_strs = [c.path for c in all.to_list()]
+    as_strs = [c.path for c in depset(transitive = all).to_list()]
     ctx.actions.write(
         output = ctx.outputs.runtime,
         content = "\n".join(sorted(as_strs)),