move rootfs to its own package

rootfs/BUILD

@@ -0,0 +1,8 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "rootfs_lib",
+    srcs = ["rootfs.go"],
+    importpath = "github.com/motiejus/code/undocker/rootfs",
+    visibility = ["//visibility:public"],
+)