Use transitive_files in runfiles instead of a list of files (#69)

diff --git a/bzl_library.bzl b/bzl_library.bzl
index 8f1a6c9..06e1b34 100644
--- a/bzl_library.bzl
+++ b/bzl_library.bzl
@@ -32,7 +32,7 @@
         # a separate program, or from `tools` of a genrule().
         DefaultInfo(
             files = all_files,
-            runfiles = ctx.runfiles(files = list(all_files)),
+            runfiles = ctx.runfiles(transitive_files = all_files),
         ),
 
         # We also define our own provider struct, for aggregation and testing.