Allow depending on files in jinja2_template

This lets us use the {% include %} directive in jinja2 to more cleanly
compose JSON files.

Change-Id: Ifdeb6f890c05ae1c20f4ffdd1540c64eb8e24be0
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/tools/build_rules/template.bzl b/tools/build_rules/template.bzl
index d4807e8..0b9167e 100644
--- a/tools/build_rules/template.bzl
+++ b/tools/build_rules/template.bzl
@@ -2,7 +2,7 @@
     out = ctx.actions.declare_file(ctx.attr.name)
 
     ctx.actions.run_shell(
-        inputs = ctx.files.src,
+        inputs = ctx.files.src + ctx.files.includes,
         tools = [ctx.executable._jinja2],
         progress_message = "Generating " + out.short_path,
         outputs = [out],
@@ -22,6 +22,10 @@
             mandatory = True,
             doc = """The parameters to supply to Jinja2.""",
         ),
+        "includes": attr.label_list(
+            allow_files = True,
+            doc = """Files which are included by the template.""",
+        ),
         "_jinja2": attr.label(
             default = "//tools/build_rules:jinja2_generator",
             cfg = "host",