fix: NVIDIA CUDA flat repos don't follow Debian repo spec

Although the Debian repo spec for 'Filename' (see
https://wiki.debian.org/DebianRepository/Format#Filename) clearly says
that 'Filename' should be relative to the base directory of the repo and
should be in canonical form (i.e. without '.' or '..') there are cases
where this is not honored.

In those cases we try to work around this by assuming 'Filename' is
relative to the sources.list directory/ so we combine them and normalize
the new 'Filename' path.

Note that, so far, only the NVIDIA CUDA repos needed this workaround so
maybe this heuristic will break for other repos that don't conform to
the Debian repo spec.

apt/private/BUILD.bazel

@@ -42,6 +42,9 @@ bzl_library(
     name = "pkg",
     srcs = ["pkg.bzl"],
     visibility = ["//apt:__subpackages__"],
+    deps = [
+        "@bazel_skylib//lib:paths",
+    ],
 )
 
 bzl_library(