refactor: Successor getting with separation of concerns

[TerryHowe]

What this PR does / why we need it:

The PrintSuccessorStatus method was complicated with the function passed in and it didn't show the best separation of concerns. The print module ideally wouldn't know about a fetcher for example.

[codecov]

Codecov Report

Attention: Patch coverage is 85.59322% with 17 lines in your changes missing coverage. Please review.

Project coverage is 83.43%. Comparing base (c31d120) to head (d9e22e8).

Files	Patch %	Lines
cmd/oras/internal/display/status/text.go	80.76%	2 Missing and 3 partials ⚠️
cmd/oras/root/cp.go	28.57%	3 Missing and 2 partials ⚠️
internal/testutils/fetcher.go	83.33%	3 Missing ⚠️
cmd/oras/internal/display/status/tty.go	90.00%	2 Missing ⚠️
cmd/oras/root/attach.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1443      +/-   ##
==========================================
+ Coverage   83.29%   83.43%   +0.13%     
==========================================
  Files         110      112       +2     
  Lines        3892     3936      +44     
==========================================
+ Hits         3242     3284      +42     
- Misses        426      428       +2     
  Partials      224      224              

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[TerryHowe]

/lgtm

[qweeah]

@TerryHowe We have changed the behavior of Println. Print error will now be returned as terminal errors. Can you help review the change, thanks.

I also tracked to-do fixes in other places in #1446

[TerryHowe]

/lgtm

I cannot approve this, but looks good

[shizhMSFT]

LGTM with suggestion. This PR also requires improved code testing. I'd suggest adding some unit tests.

[TerryHowe]

I was kind of hoping to do this as a follow up, but to increase test coverage, I moved descriptor.Successors to graph.FilteredSuccessors since it is just a specialization of graph.Successors and the framework to test it was in graph_test.go.

[TerryHowe]

There is a bit of a chicken/egg situation here. The whole reason I went down this path of refactoring successors was to create a tty handler and once the tty handler was created, the code would be way more testable. I need to think about this.

[qweeah]

LGTM, missing lines in tty.go and text.go can be covered by unit tests though.

[qweeah]

@TerryHowe Can you help add unit test to cover missing lines in cmd/oras/internal/display/status/text.go? Thanks

[qweeah]

@TerryHowe You have chosen the hard way but you have done it good! Thanks for the contribution. Added several test to cover more lines. PTAL.

[TerryHowe]

@TerryHowe You have chosen the hard way but you have done it good! Thanks for the contribution. Added several test to cover more lines. PTAL.

Thanks for working with me on this and all the others!

LGTM