fix(test): `package_naming_aggregate_test` on Windows

The test was failing on Windows:
```
==================== Test output for //tests:package_naming_aggregate_test:
ls: cannot access 'C:/[...]/bin/tests/package_naming_aggregate_test.exe.runfiles/_main/tests/test_naming_some_value.deb': No such file or directory
================================================================================
```
It tried to directly access files in the `runfiles` directory, but on
Windows Bazel uses by default a manifest file instead of creating actual
symlinks.

The change consists in adding a minimalistic version of the
[rlocation()](https://github.com/bazelbuild/rules_shell/blob/main/shell/runfiles/runfiles.bash)
helper function that:
- when set, reads from `$RUNFILES_MANIFEST_FILE` to resolve paths,
- otherwise uses `$TEST_SRCDIR` directly, as before.

This allows to enable the test in Windows CI now it passes.

Note on the stricter error handling:
- `set -u` is to fail fast should any of the expected `TEST_*`
  environment variables be unbound,
- `set -o pipefail` is to fail fast should either side of the
  `grep | cut` pipe fail.