Move GetPod from pod.go to taskrun.go

[imjasonh]

GetPod is only called from taskrun.go, so let's just move it to that
package and unexport it.

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• [y] Includes tests (if functionality changed/added)
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[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/taskrun/resources/pod.go	89.5%	89.1%	-0.4
pkg/reconciler/taskrun/taskrun.go	72.5%	73.1%	0.6

[ghost]

/lgtm

[vdemeester]

/lgtm

[tekton-robot]

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