Correctly propagate runfiles from data dependencies to gazelle (#1094)

This is a followup to #1008 which fixed this problem for the
gazelle_binary but not for the data attribute of the gazelle rule.

We are working on a python extension, where the gazelle binary will have a py_binary in its data deps.
Without this fix, the runfiles of that py_binary are absent at runtime and gazelle fails like
FileNotFoundError: [Errno 2] No such file or directory: '/home/alexeagle/.cache/bazel/_bazel_alexeagle/cdec52671ea8b1684766cfc23dcfc4be/execroot/rules_python/bazel-out/k8-fastbuild/bin/gazelle/g.runfiles/bazel_tools/tools/python/py3wrapper.sh'

def.bzl

@@ -74,9 +74,11 @@ def _gazelle_runner_impl(ctx):
     runfiles = ctx.runfiles(files = [
         ctx.executable.gazelle,
         go_tool,
-    ] + ctx.files.data).merge(
+    ]).merge(
         ctx.attr.gazelle[DefaultInfo].default_runfiles,
     )
+    for d in ctx.attr.data:
+        runfiles = runfiles.merge(d[DefaultInfo].default_runfiles)
     return [DefaultInfo(
         files = depset([out_file]),
         runfiles = runfiles,