Add 1.16 infrastructure

[dagood]

No need to review the first commit: it mechanically adds the Microsoft build infra by taking the diff of microsoft/main vs. upstream master and applying it on release-branch.go1.16. See https://github.com/microsoft/go/tree/microsoft/main/eng/doc/branches#if-microsoftmain-didnt-exist-yet for details.

The other commits should be reviewed:

• Second commit fixes CI: when I ran this branch, tests failed in moddeps_test because of eng/_util's external module dependencies: https://dev.azure.com/dnceng/internal/_build/results?buildId=1237987&view=results. This is an issue I've ran into before, and the _ prefix is meant to make moddeps_test skip the module.
  • The code that detects a _/. prefix is actually in upstream master, but not in upstream 1.16. b0c49ae introduced the skip code (along with a lot of other changes!) sometime after the 1.16 release. I backported it and marked the lines as NO MICROSOFT_UPSTREAM because upstream has no use for it without the rest of b0c49ae.

Successful official build:
https://dev.azure.com/dnceng/internal/_build/results?buildId=1238036&view=results

[dagood]

/azp run microsoft-go-pr-outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dagood]

@microsoft/golang-compiler PTAL, first commit is code flow (generated via the flow in the PR description, no need for review) and second commit is a fixup for CI.

[jcouv]

LGTM Thanks (iteration 2)