fietsje: implement .replace, add Cilium dependencies.

The new .replace() can be used to mirror 'replace' stanzas in go.mod,
and that's what it's being used for in Cilium, as it ships a handful of
forked libraries that we have to pull in.

The Cilium targets are currently unused, but the ones confirmed to build
are:

 - @com_github_cilium_cilium//cilium: cilium API client
 - @com_github_cilium_cilium//daemon:daemon: cilium daemon/agent
 - @com_github_cilium_cilium//operator: cilium operator

These currently built as dynamic libraries - turning them into
static/pure builds will come in a later build.

Test Plan: how do we test this? :)

X-Origin-Diff: phab/D542
GitOrigin-RevId: b38c7c1d0be8b0b88ea8f6992c9c5557189399cc
diff --git a/build/fietsje/BUILD.bazel b/build/fietsje/BUILD.bazel
index b54b307..4a5d3fb 100644
--- a/build/fietsje/BUILD.bazel
+++ b/build/fietsje/BUILD.bazel
@@ -4,6 +4,7 @@
     name = "go_default_library",
     srcs = [
         "dependency.go",
+        "deps_cilium.go",
         "deps_containerd.go",
         "deps_gvisor.go",
         "deps_kubernetes.go",