Use library namespace by default for docker.io images

[jonesbusy]

Description

Use library namespace by default for docker.io images

Also found during interactive tests that multi-arch images are not exported (since an index is returned). An other PR will follow

Testing done

CI and interactive tests with docker.io

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.05%. Comparing base (ee0d198) to head (bbcfead).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/Registry.java	80.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #181      +/-   ##
============================================
+ Coverage     84.87%   85.05%   +0.17%     
- Complexity      335      340       +5     
============================================
  Files            29       29              
  Lines          1164     1171       +7     
  Branches        113      115       +2     
============================================
+ Hits            988      996       +8     
+ Misses          122      121       -1     
  Partials         54       54              

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