Move `github.com/fluxcd/source-controller/pkg` to either `source-controller/internal/bucket` or `fluxcd/pkg/bucket`

The source-controller is the only Flux controller exposing a public `/pkg` besides `/api`. This package has code for accessing Bucket providers. To make it aligned with everything else, we should decide on either moving it to `fluxcd/pkg/bucket` and continue supporting it as a GA API (starting from `v1.0.0`), or moving it to `source-controller/internal/bucket`.