Detect unintentional multiple *.config.php files to catch weird config problems  #46665

Sometimes an administrator makes a manual backup of their existing config before making an adjustment. If they name it {anything}.config.php this results in them unintentionally having overlapping config files. And it also means the backup takes precedence. This creates all sorts of unexpected behaviors that can be difficult to isolate (and may give the appearance of bugs)¹.

Multiple config.php file support is valid so this problem is difficult to detect automatically without creating lots of false positives. I think we can take a pragmatic approach by focusing on two areas of improvement:

• logging / indicating when multiple config files are in-use in a few spots (where it won't be overly noisy/intrusive in the environments where it's perfectly valid)
• detect the most common yet least likely to be a false positive situation: multiple config files with overlapping version field values

Specifically:

• during the upgrade we log that multiple config files were detected (not an error, just notes it so it'll be obvious when logs are reviewed or provided for troubleshooting)
• create a setup check that detects obvious signs of a probable unintentional multi-config file situation.²
• possibly we give an indication that multiple config files are in-use somewhere in the UI
• possibly we give an indication that multiple config files are in-use when running some occ commands³