go.mod: update to github.com/cyphar/filepath-securejoin@v0.5.0

In order to avoid lint errors due to the deprecation of the top-level
securejoin methods ported from libpathrs, we need to adjust
internal/pathrs to use the new pathrs-lite subpackage instead.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

vendor/github.com/cyphar/filepath-securejoin/README.md

@@ -67,7 +67,8 @@ func SecureJoin(root, unsafePath string) (string, error) {
 [libpathrs]: https://github.com/openSUSE/libpathrs
 [go#20126]: https://github.com/golang/go/issues/20126
 
-### New API ###
+### <a name="new-api" /> New API ###
+[#new-api]: #new-api
 
 While we recommend users switch to [libpathrs][libpathrs] as soon as it has a
 stable release, some methods implemented by libpathrs have been ported to this
@@ -165,5 +166,19 @@ after `MkdirAll`).
 
 ### License ###
 
-The license of this project is the same as Go, which is a BSD 3-clause license
-available in the `LICENSE` file.
+`SPDX-License-Identifier: BSD-3-Clause AND MPL-2.0`
+
+Some of the code in this project is derived from Go, and is licensed under a
+BSD 3-clause license (available in `LICENSE.BSD`). Other files (many of which
+are derived from [libpathrs][libpathrs]) are licensed under the Mozilla Public
+License version 2.0 (available in `LICENSE.MPL-2.0`). If you are using the
+["New API" described above][#new-api], you are probably using code from files
+released under this license.
+
+Every source file in this project has a copyright header describing its
+license. Please check the license headers of each file to see what license
+applies to it.
+
+See [COPYING.md](./COPYING.md) for some more details.
+
+[umoci]: https://github.com/opencontainers/umoci