Credential implementation files are not independently requireable

## Summary

Several documented implementation files rely on load order from the aggregate `googleauth` entrypoint. Four focused requires fail on current 1.17.4/`main` (`01431c9ecd59e1efc5d262694971ded925044fdc`): external AWS, identity-pool and pluggable credentials are missing `CredentialsLoader`, while the service-account JWT header is missing `BaseClient`. Two external-account files also accidentally append `require "time"` to the license-comment line, so it never executes.

## Proposed correction

Add each direct dependency require and restore the comment/newline boundary. Candidate: `02fc41cca34504faaaadb4a01ffdd7cb864df01d`.

All 37 runtime files load independently with the candidate. The cumulative suite passes 659 RSpec examples (20 expected pending), 103 Minitest runs /201 assertions and four integration runs /41 assertions, plus RuboCop, 86-file syntax, YARD, package/install parity and all five Songstats consumers.

I used an AI coding assistant during source review and verification. I am opening an issue first because repository policy requires tests, a changelog and a Google CLA for pull requests; this audit cannot alter repository tests and no CLA was signed as part of it.