fix(aptpkg): change glob to only match top-level .list files

[nepeat]

What does this PR do?

This changes the glob pattern for APT sources from **/*.list to *.list, in line APT's default pattern in sources.list.d

What issues does this PR fix or reference?

Fixes #68475

Previous Behavior

Exception is thrown if there is a .list file in a subdirectory (e.g: backups/private.list)

New Behavior

No exception is thrown

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [X] Tests written/updated

Commits signed with GPG?

No