Refactoring resource list

[piyush-garg]

This will refactor the listing of pipeline resource
in tkn res list and tkn task describe taskname
and tkn pipeline describe pipelinename commands.

Now it will show the resources sorted by type and
then further sorted by name

Refactor tests accordingly

Fixes #148

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These are the criteria that every PR should meet, please check them off as you
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• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
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[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/cmd/pipelineresource/list.go	85.0%	87.0%	2.0
pkg/cmd/task/describe.go	89.7%	92.5%	2.8

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/cmd/pipeline/describe.go	87.5%	90.9%	3.4
pkg/cmd/pipelineresource/list.go	85.0%	87.0%	2.0
pkg/cmd/task/describe.go	89.7%	92.5%	2.8

[chmouel]

/lgtm
/approve

[tekton-robot]

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