osbase/build: move efi.bzl, split and move def.bzl to their corresponding action

This is a small reorganization to make the osbase/build less dependent on each other.

Change-Id: I8c12f04f3bdc98128c5424f142f452c2e094f2e8
Reviewed-on: https://review.monogon.dev/c/monogon/+/3903
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/installer/test/BUILD.bazel b/metropolis/installer/test/BUILD.bazel
index 7390cf6..7f7ad54 100644
--- a/metropolis/installer/test/BUILD.bazel
+++ b/metropolis/installer/test/BUILD.bazel
@@ -1,5 +1,5 @@
 load("@io_bazel_rules_go//go:def.bzl", "go_test")
-load("//osbase/build:efi.bzl", "efi_unified_kernel_image")
+load("//osbase/build/mkpayload:def.bzl", "efi_unified_kernel_image")
 
 go_test(
     name = "test_test",