Roll out new unified standards to Source controller

[scottrigby]

• Part of Roll out new unified standards to all Flux controllers flux2#1601

This issue is to track rolling out the changes to Source Controller.

The current strategy is one PR per reconcilier within each controller:

• Rewrite GitRepositoryReconciler to new standards  #411
• Rewrite BucketReconciler to new standards #412
• Rewrite HelmRepositoryReconciler to new standards #413
• Forthcoming PR: Rewrite HelmChartReconciler to new standards @hiddeco
• Forthcoming PR: deal with the implementation of External event recorder should adhere to EventRecorder interface pkg#134, Mark events to prevent alerting spam pkg#139