Refactor CI ProjectConfig sources to use static precedence
What does this MR do and why?
This change refactors sources for ProjectConfig
to use a static list of sources instead of manipulating it in EE to make the precedence of sources clearer.
EE-only configs return nil
for CE.
The MR seems quite big, but it's mostly just moving existing code around.
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Related to #466430 (closed)