pin-dependency.sh: enhance forking of a dependency

[pohly]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This addresses a problem that occurred when trying the following replacement:

   hack/pin-dependency.sh k8s.io/klog/v2=github.com/pohly/klog/v2 flush-log-sink

That itself worked, but update-vendor.sh then failed:

   === tidying go.mod/go.sum in staging/src/k8s.io/code-generator/examples
  go: k8s.io/apimachinery@v0.0.0 requires
  	k8s.io/klog/v2@v2.4.1-0.20210909144920-0b91caa8c854: invalid version: unknown revision 0b91caa8c854
  go: k8s.io/apimachinery@v0.0.0 requires
  	k8s.io/klog/v2@v2.4.1-0.20210909144920-0b91caa8c854: invalid version: unknown revision 0b91caa8c854

Special notes for your reviewer:

Not sure why that problem surfaced now and not when I originally added the support for forking a dependency.

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

@pohly: This issue is currently awaiting triage.

If a SIG or subproject determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

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[pohly]

/assign @liggitt

[liggitt]

/sig architecture
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, pohly

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