refactor: Minor improvements to BackendConfigLoader

[dave-gray101]

some minor renames and refactorings within BackendConfigLoader:

• make things more consistent in receiver variable naming
• remove underused code to centralize things
• drop public exports where possible
• rename things for clarity - some of these are a bit long but the goal was to require less contextual information to know what is being loaded or read.

Description

This PR fixes a couple annoyances I had with backend_config_loader.go

Notes for Reviewers Should be no functional changes, existing tests appear sufficient to cover what I'm touching here.

Signed commits

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