Define scripts/ as a submodule

[chizhg]

What this PR does, why we need it:
I did some experiment with Go multiple modules to vendor scripts/, and did not see any issues.

The only changes needed in each repo will be changing the floating deps from knative.dev/test-infra to knative.dev/test-infra/scripts in for example https://github.com/knative/serving/blob/master/hack/update-deps.sh#L35. And even if it's not changed, it can still work in the old way.

Which issue(s) this PR fixes:
Fixes #2078

/cc @chaodaiG @albertomilan

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[chaodaiG]

Looks promising, can you fix the build error first?

[chizhg]

Looks promising, can you fix the build error first?

I didn't find an elegant way to fix the build error. It seems go build -v ./... cannot work with a repo that has multiple Go modules and will get an error go: directory scripts is outside main module, I'm not getting the error when I run the command locally though...

I think we can just skip building the submodule it contains mainly scripts, and since we are already skipping a few directories, the approach is not that dirty.

[chaodaiG]

Looks promising, can you fix the build error first?

I didn't find an elegant way to fix the build error. It seems go build -v ./... cannot work with a repo that has multiple Go modules and will get an error go: directory scripts is outside main module, I'm not getting the error when I run the command locally though...

I think we can just skip building the submodule it contains mainly scripts, and since we are already skipping a few directories, the approach is not that dirty.

I couldn't repro the error locally either, maybe we should figure out why this happened instead of going straight to the workaround?

[chaodaiG]

/hold

[chizhg]

Looks promising, can you fix the build error first?

I didn't find an elegant way to fix the build error. It seems go build -v ./... cannot work with a repo that has multiple Go modules and will get an error go: directory scripts is outside main module, I'm not getting the error when I run the command locally though...
I think we can just skip building the submodule it contains mainly scripts, and since we are already skipping a few directories, the approach is not that dirty.

I couldn't repro the error locally either, maybe we should figure out why this happened instead of going straight to the workaround?

I can reproduce the error by running ./test/presubmit-tests.sh --build-tests. The error happens in running command go test -c -tags="e2e library local tools" ./scripts.

There are two issues with this command:

1. Currently the build test runner is assuming all packages that have build tags are test packages, but it's not necessarily true.
2. As we are now defining ./scripts as a go submodule, even go build -tags="tools" ./scripts will get the go: directory scripts is outside main module error, the right way to build it will be cd scripts/ && go build -tags="tools" ..

Given the first issue and the fact the submodule only contains a dummy.go file, I think it does make sense for us to explicitly skip building it for now. I have added a TODO to support building normal go packages with build tags if we need it in the future.

[chaodaiG]

/lgtm

[chizhg]

/hold cancel