petite DAG - introduce key to identify node instead of holding a task

[pritidesai]

Changes

A graph is built using all pipelineTasks from tasks section as nodes. A node includes a pipelineTask along with a list of parents (prev) and dependencies(next). The entire pipelineTask struct is not needed in building a graph since the list of dependencies is calculated outside of dag.build and passed as an additional parameter. This struct (pipelineTask) doesn't serve any purpose and not required in the node. The node now includes a key instead of Task.

For example, the DAG for this unit test now shows:

 Nodes: (map[string]*dag.Node) (len=6) {
  (string) (len=1) "a": (*dag.Node)(0xc000467900)({
   Key: (string) (len=1) "a",
   Prev: ([]*dag.Node) <nil>,
   Next: ([]*dag.Node) (len=2 cap=2) {
    (*dag.Node)(0xc000467940)({
     Key: (string) (len=1) "x",
     Prev: ([]*dag.Node) (len=1 cap=1) {
      (*dag.Node)(0xc000467900)(<already shown>)
     },
...

Instead of:

 Nodes: (map[string]*dag.Node) (len=6) {
  (string) (len=1) "a": (*dag.Node)(0xc0005ab780)({
   Task: (v1beta1.PipelineTask) {
    Name: (string) (len=1) "a",
    TaskRef: (*v1beta1.TaskRef)(<nil>),
    TaskSpec: (*v1beta1.EmbeddedTask)(<nil>),
    WhenExpressions: (v1beta1.WhenExpressions) <nil>,
    Retries: (int) 0,
    RunAfter: ([]string) <nil>,
    Resources: (*v1beta1.PipelineTaskResources)(<nil>),
    Params: ([]v1beta1.Param) <nil>,
    Matrix: (*v1beta1.Matrix)(<nil>),
    Workspaces: ([]v1beta1.WorkspacePipelineTaskBinding) <nil>,
    Timeout: (*v1.Duration)(<nil>)
   },
   Prev: ([]*dag.Node) <nil>,
   Next: ([]*dag.Node) (len=2 cap=2) {
    (*dag.Node)(0xc0005ab7c0)({
     Task: (v1beta1.PipelineTask) {
      Name: (string) (len=1) "x",
      TaskRef: (*v1beta1.TaskRef)(<nil>),
      TaskSpec: (*v1beta1.EmbeddedTask)(<nil>),
      WhenExpressions: (v1beta1.WhenExpressions) <nil>,
      Retries: (int) 0,
      RunAfter: ([]string) <nil>,
      Resources: (*v1beta1.PipelineTaskResources)(0xc00050d560)({
       Inputs: ([]v1beta1.PipelineTaskInputResource) (len=1 cap=1) {
        (v1beta1.PipelineTaskInputResource) {
         Name: (string) "",
         Resource: (string) "",
         From: ([]string) (len=1 cap=1) {
          (string) (len=1) "a"
         }
        }
       },
       Outputs: ([]v1beta1.PipelineTaskOutputResource) <nil>
      }),
      Params: ([]v1beta1.Param) <nil>,
      Matrix: (*v1beta1.Matrix)(<nil>),
      Workspaces: ([]v1beta1.WorkspacePipelineTaskBinding) <nil>,
      Timeout: (*v1.Duration)(<nil>)
     },
     Prev: ([]*dag.Node) (len=1 cap=1) {
      (*dag.Node)(0xc0005ab780)(<already shown>)
     },
...

/kind cleanup

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
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Release Notes

Replace holding a `Task` in `dag.Node` with a unique string identifier. 

[abayer]

/test check-pr-has-kind-label

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[abayer]

/approve

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[dibyom]

/lgtm

[jerop]

/test tekton-pipeline-unit-tests

cloud events flake 🤔

[pritidesai]

thank you @abayer, @dibyom, and @jerop for reviews 👍

Unit tests failing with the known cloud event flake - #5160 😞

/test tekton-pipeline-unit-tests