Support overriding go_sdk versions

[justaugustus]

(Suggested in #197 (comment).)

I've roughly borrowed this pattern from rules_go (h/t @jayconrod).
See: https://github.com/bazelbuild/rules_go/blob/9dc42edc13c67d8a70a14e262a7b128ed26a3e54/go/private/sdk.bzl#L23-L28 and https://github.com/bazelbuild/rules_go/blob/9dc42edc13c67d8a70a14e262a7b128ed26a3e54/go/private/sdk_list.bzl#L19

Here we extend the configure function in repos.bzl to accept an override_go_version parameter.

If override_go_version is specified, configure will make a call to go_download_sdk as follows:

go_download_sdk(name = "go_sdk", sdks = OVERRIDE_GO_VERSIONS[override_go_version])

OVERRIDE_GO_VERSIONS is a map that can contain multiple alternate go_sdk versions.
This PR populates the map with go1.15beta1 / 1.15.0-beta.1, which can be used to test the kubernetes/kubernetes upgrade to go1.15 in kubernetes/kubernetes#93264.

A consumer can leverage this new functionality as follows:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "io_k8s_repo_infra",
    strip_prefix = "repo-infra-0.0.8",
    sha256 = "2a86a60706c1f9b2d87f25d2bc1d107d823908ec3fb895735c8aa9a4b239d10e",
    urls = [
        "https://github.com/kubernetes/repo-infra/archive/v0.0.8.tar.gz",
    ],
)

load("@io_k8s_repo_infra//:load.bzl", repo_infra_repositories = "repositories")

repo_infra_repositories()

load("@io_k8s_repo_infra//:repos.bzl", repo_infra_configure = "configure", repo_infra_go_repositories = "go_repositories")

repo_infra_configure(
    override_go_version = "1.15.0-beta.1",
    minimum_bazel_version = "2.2.0",
)

repo_infra_go_repositories()

You can see a test of this functionality in kubernetes/kubernetes#93395.

[justaugustus]

/check-cla

[justaugustus]

/hold
/assign @fejta @liggitt @Verolop
cc: @kubernetes/release-engineering

[fejta]

Any reason to put this in a different file rather than in here?

[justaugustus]

@fejta -- Mostly long-term maintenance. This puts us on a path to support Go pre-releases and that list is going to get pretty long. I didn't want to clutter up our primary logic with all of the pre-release tuples.

[k8s-ci-robot]

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[justaugustus]

/hold cancel