Check debug session for multiple container in step.

[sudhirverma]

Fix: #640

[lstocchi]

The problem I can see is that we show all steps together within the terminal and the last one is focussed. So if you don't pay attention to the container you are in, you may execute the debug script in a container which is not the one used currently by tekton.
E.g i have a task with three steps. The first step works fine, the other two should fail because they try to run a not-existing npm command.
When i start debug on this task i see

When defining steps in a task, tekton executes them in their order, so first -> second -> third in my case.
There is no reason to show to the user all running containers, we should show user only the step which is ready to be debugged, so the first one which is running after the last completed. In my case, the second step, because the first has been completed and the second is running. This is necessary because, maybe, the previous step is used to set up something for the following. Let's say i'm downloading some files somewhere in the second step and they are used in the third. Why should we allow users to get inside the third step if the second is not completed?

I guess this is the only thing to be improved because if I execute the script in the right order: second step -> third step, it works fine.

Other thought for the future is to switch watching taskruns to pods.

1. This gives you a protection against problems that tekton is not able to check. The same reason we created the diagnostic action E.g when a pod doesn't start because it can't create a pvc or other errors like this. I think that watching taskrun doesn't help with it. You will see taskrun in running state when the pod will never be created. Not sure though.
2. Other advantage is that you can re-use the code for pipeline when there will be debug for it as well.

[codecov-commenter]

Codecov Report

Merging #646 (ef32826) into master (7faf1da) will decrease coverage by 0.00%.
The diff coverage is 41.66%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
- Coverage   68.33%   68.33%   -0.01%     
==========================================
  Files         114      114              
  Lines        6550     6553       +3     
  Branches     1152     1153       +1     
==========================================
+ Hits         4476     4478       +2     
- Misses       2074     2075       +1     

Impacted Files	Coverage Δ
src/debugger/debug-tree-view.ts	59.52% <31.57%> (+0.54%)	⬆️
src/cli-command.ts	74.05% <50.00%> (ø)
src/debugger/show-in-terminal.ts	92.85% <100.00%> (ø)

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[lstocchi]

LGTM, tested with the pipeline demo, works fine.