fix: properly print tree for oras discover

[shizhMSFT]

What this PR does / why we need it:

• Properly print tree for oras discover
• Remove the dependency of github.com/need-being/go-tree, which is no longer maintained and does not have any tests

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1003

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov-commenter]

Codecov Report

Merging #1005 (9064314) into main (42650e5) will increase coverage by 0.01%.
The diff coverage is 78.94%.

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@@            Coverage Diff             @@
##             main    #1005      +/-   ##
==========================================
+ Coverage   81.14%   81.16%   +0.01%     
==========================================
  Files          55       57       +2     
  Lines        2848     2904      +56     
==========================================
+ Hits         2311     2357      +46     
- Misses        367      373       +6     
- Partials      170      174       +4     

Impacted Files	Coverage Δ
internal/tree/printer.go	60.00% <60.00%> (ø)
cmd/oras/root/discover.go	85.21% <100.00%> (ø)
internal/tree/node.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[shizhMSFT]

Note: @need-being is my personal GitHub account

[TerryHowe]

/lgtm

[qweeah]

LGTM