How to set two go_download_sdk rules and switch between them

We have two versions of Golang: the official version and a modified/packaged version. For example, we need to use the ```bazel --patchgo``` command to compile a service using the modified/packaged version of Golang, and if the ```--patchgo``` command is not used, we need to use the official version of Golang to compile the service. How can we configure this?

Start by reviewing how rules_go exposes go_download_sdk configuration and how the bazel --patchgo invocation is expected to select a toolchain. Reproduce the official-versus-modified Go scenario, then document a supported configuration or clearly state that this switching behavior is not supported.

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