Install external hack/tools with a temporary go module

[sbueringer]

We currently install all our hack/tool's via the hack/tools/go.mod go module. After a discussion in Slack (https://kubernetes.slack.com/archives/C8TSNPY4T/p1635440307207400) the consensus was that we want to install the tools via individual temporary go modules.

An example can be seen in CAPG: https://github.com/kubernetes-sigs/cluster-api-provider-gcp/blob/main/Makefile#L187-L188

Essentially, we ~ need the ./scripts/go_install.sh script and then the following in our Makefile (example kustomize):

KUSTOMIZE_VER := v3.8.6
KUSTOMIZE_BIN := kustomize
KUSTOMIZE := $(TOOLS_BIN_DIR)/$(KUSTOMIZE_BIN)-$(KUSTOMIZE_VER)

[...]
$(KUSTOMIZE): ## Build kustomize from tools folder.
	GOBIN=$(TOOLS_BIN_DIR) $(GO_INSTALL) sigs.k8s.io/kustomize/kustomize/v3 $(KUSTOMIZE_BIN) $(KUSTOMIZE_VER)

I think the following tools can be installed this way:

• setup-envtest
• controller-gen
• gotestsum
• conversion-gen
• go-apidiff
• envsubst
• yq
• kustomize (should replace the shell script, as the shell script doesn't enforce the kustomize version)
• golangci-lint (not sure about this one)

This helps us to avoid a monolithic hack/tools/go.mod file and after this change it only contains the dependencies of our local hack/tool's

Also this helps to make the version of our dependencies visible top-level (in the Makefile instead of hack/tools/go.mod)

/kind feature

[sbueringer]

/area code-organization
/help

[k8s-ci-robot]

@sbueringer:
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[kaitoii11]

/assign

[fabriziopandini]

/milestone v1.2