Refactored WP_Debug_Data::debug_data() - get_wp_mu_plugins()

[apermo]

Changed: Moving $info['wp-mu-plugins'] to own function

I've started a second parallel PR, a little lower, so that we should not get too many merge conflicts.

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Follow up to #7027 as discussed with @dmsnell and @costdev

Summary of the coming steps:

1. wp-filesystem
2. wp-constants
3. wp-database
4. wp-server
5. wp-media
6. wp-plugins-active & wp-plugins-inactive
7. wp-mu-plugins (we are here)
8. wp-themes-inactive
9. wp-parent-theme
10. wp-active-theme
11. wp-dropins
12. wp-paths-sizes
13. wp-core

Refactoring WP_Debug_Data::debug_data(); into smaller functions for

• better maintainability
• reduced complexity
• preparation for future unit tests
• improved extensibility (see trac for details)
• added php7.2 style type hints
• fixed an error with $parent_theme_auto_update_string showing the value from $auto_updates_string (This will be handled only in Step 9)

Trac ticket: https://core.trac.wordpress.org/ticket/61648#ticket

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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Props apermo, dmsnell.

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[apermo]

@dmsnell since we have our structure now, I've opened a second PR, I've opted to have it a little in the middle, so that we can easily rebase if needed with hopefully little to no merge conflicts.

[github-actions]

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[dmsnell]

@apermo I've merged trunk into this and hope I did that properly. how does it look? is this ready in your opinion for merging?

[apermo]

@apermo I've merged trunk into this and hope I did that properly. how does it look? is this ready in your opinion for merging?

Looks good to me. @dmsnell

[dmsnell]

Merged in [59011]
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