Clean up pipelinerun resolution code

[lbernick]

Pipelinerun resolution code (e.g. https:/tektoncd/pipeline/blob/main/pkg/reconciler/pipelinerun/resources/pipelinerunresolution.go) is getting super hard to understand, making it challenging to evaluate code that modifies it (e.g. #6603 (comment)).

Some specific changes that would go a long way for readability:

• Adding docstrings for functions in the resources package, especially pipelinerunresolution.go
• Removing redundant information from ResolvedPipelineTask. For example, this struct contains "TaskRun", "TaskRuns", "TaskRunName", and "TaskRunNames". It's hard to know which of these fields code should use.
• Renaming references to "resources". I suspect this originally referred to PipelineResources, and expanded over time to refer to resolving things like tasks, params, workspaces etc etc.
• Try to avoid overloading the word "resolve". This is currently used to refer to fetching remote tasks, getting all the taskruns from a pipeline task, getting results from previous tasks, etc.

Another aspect of ResolvedPipelineTask that's confusing is that it's hard to know at what point in the code you can expect referenced TaskRuns and CustomRuns to be non-nil. For example, if ResolvedPipelineTask.TaskRun is nil, does that mean there is no taskrun associated with the pipeline task yet, or just that we haven't yet fetched it from the cluster? (Or that it's a matrixed pipeline task, and uses TaskRuns instead?)

cc @EmmaMunley

[EmmaMunley]

/assign

[lbernick]

between #6643 and #6649 I think we're good to close this!