feat(repositories): stop removing sources files and support unmanaged

[didiermfb]

Based on sir-ragna's work on the PR #77
I think it's better to split the PR

PR progress checklist (to be filled in by reviewers)

• Changes to documentation are appropriate (or tick if not required)
• Changes to tests are appropriate (or tick if not required)
• Reviews completed

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What type of PR is this?

Primary type

• [build] Changes related to the build system
• [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ci] Changes to the continuous integration configuration
• [feat] A new feature
• [fix] A bug fix
• [perf] A code change that improves performance
• [refactor] A code change that neither fixes a bug nor adds a feature
• [revert] A change used to revert a previous commit
• [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [docs] Documentation changes
• [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No.

Related issues and/or pull requests

Describe the changes you're proposing

When setting apt:clean_sources_list_d=true files that are not required to be removed are not removed
A feature is added so that adding apt:repositories:repo:unmanaged=true allows you to specifically exclude a sources.list file from getting removed (or managed at all). This is handy in case another formula is managing this file.

Pillar / config required to test the proposed changes

Debug log showing how the proposed changes work

Documentation checklist

• Updated the README (e.g. Available states).
• Updated pillar.example.

Testing checklist

• Included in Kitchen (i.e. under state_top).
• Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• Updated the relevant test pillar.

Additional context

Added a Salt state to the tests to verify it isn't removed.

repos_maintained_by_another_formula:
  file.managed:
    - name: /etc/apt/sources.list.d/unmanaged.list
    - mode: '0644'
    - contents: "## unmanged list file that shouldn't be removed"

[didiermfb]

Rebased done.
Currently if the repo file is not updated, salt doesn't take care of the key. There is an open issue about it : saltstack/salt#66982
If you moved some of your lists to the new deb822 format and you want them to be unmanaged, you need to use the filename. Ex:

apt:
  repositories:
    deb822_repo:
      unmanaged: true
      filename: deb822_repo.sources

[javierbertoli]

It LGTM. Great work, @didiermfb !