fix: remove root from virtual referrers

[shizhMSFT]

What this PR does / why we need it:
Fix a bug when oras cp -r a loop index.

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 #1129

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 #1130 (89f4bed) into main (8cf946f) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@            Coverage Diff             @@
##             main    #1130      +/-   ##
==========================================
- Coverage   80.10%   80.06%   -0.05%     
==========================================
  Files          57       57              
  Lines        2991     2994       +3     
==========================================
+ Hits         2396     2397       +1     
- Misses        410      412       +2     
  Partials      185      185              

Files Changed	Coverage Δ
cmd/oras/root/cp.go	81.36% <100.00%> (+0.35%)	⬆️

... and 1 file with indirect coverage changes

[sajayantony]

Should we include a test for this case?

[qweeah]

LGTM. BTW is this allowed by spec? Having a subject which is also a member of manifests sound wierd.

[qweeah]

Should we include a test for this case?

I can add one